The standard release of Drupal, known as Drupal core, contains basic features common to content management systems. These include user account registration and maintenance, menu management, RSS feeds, taxonomy, page layout customization, and system administration. The Drupal core installation can be used as a simple website, a single- or multi-user blog, an Internet forum, or a community website providing for user-generated content.
As of February 2014[update], there are more than 30,000 free community-contributed addons, known as contributed modules, available to alter and extend Drupal's core capabilities and add new features or customize Drupal's behavior and appearance. Also the Drupal community has more than 1 million members  (As of October 2013[update]) and 31,000 Developers (As of February 2014[update]). Because of this plug-in extensibility and modular design, "The Drupal Overview" on Drupal's website describes it as a content management framework. Drupal is also described as a web application framework, as it meets the generally accepted feature requirements for such frameworks.
Although Drupal offers a sophisticated programming interface for developers, no programming skills are required for basic website installation and administration.
Originally written by Dries Buytaert as a message board, Drupal became an open-source project in 2001. The name Drupal represents an English rendering of the Dutch word "druppel", which means "drop" (as in "a water droplet"). The name came from the now-defunct Drop.org website, whose code slowly evolved into Drupal. Buytaert wanted to call the site "dorp" (Dutch for "village") for its community aspects, but mistyped it when checking the domain name and thought the error sounded better.
Interest in Drupal got a significant boost in 2003 when it helped build "DeanSpace" for Howard Dean, one of the candidates in the U.S. Democratic Party's primary campaign for the 2004 U.S. presidential election. DeanSpace used open-source sharing of Drupal to support a decentralized network of approximately 50 disparate, unofficial pro-Dean websites that allowed users to communicate directly with one another as well as with the campaign. After Dean ended his campaign, members of his web team continued to pursue their interest in developing a web platform that could aid political activism by launching CivicSpace Labs in July 2004, "the first company with full-time employees that was developing and distributing Drupal technology". Other companies began to also specialize in Drupal development. By 2013 the Drupal website listed hundreds of vendors that offered Drupal-related services.
As of 2014[update] Drupal is developed by a community, and its popularity is growing rapidly. From July 2007 to June 2008 the Drupal.org website provided more than 1.4 million downloads of drupal software, an increase of approximately 125% from the previous year.
As of February 2014[update] more than 1,015,000 sites used Drupal. These include hundreds of well-known organizations, including corporations, media and publishing companies, governments, non-profits, schools, and individuals. Drupal has won several Packt Open Source CMS Awards and won the Webware 100 three times in a row.
On March 5, 2009 Buytaert announced a code freeze for Drupal 7 for September 1, 2009. Drupal 7 was released on January 5, 2011, with release parties in several countries. After that, maintenance on Drupal 5 stopped, with only Drupal 7 and Drupal 6 maintained. Drupal 7 series maintenance updates are released regularly.
Drupal 8 is in development, with no set release date yet. The work on Drupal 8 is divided into categories, called Core initiatives: Mobile, Layouts, Web Services, Configuration Management, and HTML5. Google Summer of Code is sponsoring 20 Drupal projects.
In the Drupal community, the term "core" means anything outside of the "sites" folder of a Drupal installation. Drupal core is the stock element of Drupal. In a Drupal website's default configuration, content can be contributed by either registered or anonymous users (at the discretion of the administrator) and is made accessible to web visitors by a variety of selectable criteria. Drupal core also includes a hierarchical taxonomy system, which allows content to be categorized or tagged with key words for easier access.
Drupal maintains a detailed changelog of core feature updates by version.
Drupal core includes optional modules that can be enabled by the administrator to extend the functionality of the core website.
The core Drupal distribution provides a number of features, including:
Access statistics and logging
Blogs, books, comments, forums, and polls
Caching and feature throttling for improved performance
Drupal can automatically notify the administrator about new versions of modules, themes, or the Drupal core. Such a feature can be useful for security fixes.
Prior to version 7, Drupal had functions that performed tasks related to databases, such as SQL query cleansing, multi-site table name prefixing, and generating proper SQL queries. In particular, Drupal 6 introduced an abstraction layer that allowed programmers to create SQL queries without writing SQL.
Drupal 7 extends the data abstraction layer so that a programmer no longer needs to write SQL queries as text strings. It uses PHP Data Objects to abstract the physical database. Microsoft has written a database driver for their SQL Server. Drupal 7 supports the file-based SQLite database engine, which is part of the standard PHP distribution.
Embracing Windows developers
With Drupal 7's new database abstraction layer and ability to run on the Windows web server IIS, it is now easier for Windows developers to participate in the Drupal community. A group on Drupal.org is dedicated to Windows issues.
With the release of Drupal 7, web accessibility has been greatly improved by the Drupal community. Drupal is a good framework for building websites accessible to people with disabilities because many of the best practices have been incorporated into the program code Core. The accessibility team is carrying on the work of identifying and resolving accessibility barriers and raising awareness within the community. Drupal 7 started the adoption of WAI-ARIA support for Rich Internet Applications and this has been carried further in Drupal 8. There have been many improvements to both the visitor and administrator sides of Drupal, especially:
Drag and drop functionality
Improved color contrast and intensity
Adding skip navigation to Core themes
Adding labels by default for input forms
Fixing CSS display:none with consistent methods for hiding & exposing text onfocus.
The community also added an accessibility gate for Core issues in Drupal 8.
Extending the core
Drupal core is modular, defining a system of hooks and callbacks, which are accessed internally via an API. This design allows third-party contributed modules and themes to extend or override Drupal's default behaviors without changing Drupal core's code.
Drupal isolates core files from contributed modules and themes. This increases flexibility and security and allows administrators to cleanly upgrade to new releases without overwriting their site's customizations. The Drupal community has the saying "Never hack core", a strong recommendation that site developers do not change core files.
Contributed modules offer such additional or alternate features as image galleries, custom content types and content listings, WYSIWYG editors, private messaging, third-party integration tools, and more. As of February 2014[update] the Drupal website lists more than 25,500 free modules.
Some of the most commonly used contributed modules include:
Content Construction Kit (CCK): allows site administrators to dynamically create content types by extending the database schema. "Content type" describes the kind of information. Content types include, but are not limited to, events, invitations, reviews, articles, and products. The CCK Fields API is in Drupal core in Drupal 7.
Views: facilitates the retrieval and presentation, through a database abstraction system, of content to site visitors.
Panels: drag and drop layout manager that allows site administrators to visually design their site.
Contributed themes adapt or replace a Drupal site's default look and feel.
Drupal themes use standardized formats that may be generated by common third-party theme design engines. Many are written in the PHPTemplate engine or, to a lesser extent, the XTemplate engine. Some templates use hard-coded PHP. Drupal 8 will integrate the Twig templating engine.
The inclusion of the PHPTemplate and XTemplate engines in Drupal addressed user concerns about flexibility and complexity. The Drupal theming system utilizes a template engine to further separate HTML/CSS from PHP. A popular Drupal contributed module called 'Devel' provides GUI information to developers and themers about the page build.
Community-contributed themes at the Drupal website are released under a free GPL license, and most of them are demonstrated at the Drupal Theme Garden.
In the past, those wanting a fully customized installation of Drupal had to download a pre-tailored version separately from the official Drupal core. Today, however, a distribution defines a packaged version of Drupal that upon installation, provides a website or application built for a specific purpose.
The distributions offer the benefit of a new Drupal site without having to manually seek out and install third-party contributed modules or adjust configuration settings. They are collections of modules, themes, and associated configuration settings that prepare Drupal for custom operation. For example, a distribution could configure Drupal as a "brochureware" site rather than a "news" site or an "online store".
Drupal is based on less publicized but still widely used architecture Presentation Abstraction Control, or PAC. The menu system acts as the Controller. It accepts input via a single source (HTTP GET and POST), routes requests to the appropriate helper functions, pulls data out of the Abstraction (nodes and, from Drupal 5 onwards, forms), and then pushes it through a filter to get a Presentation of it (the theme system). It even has multiple, parallel PAC agents in the form of blocks that push data out to a common canvas (page.tpl.php).
Drupal.org has a large community of users and developers, with over 1,015,000 user accounts and over 31,000 developer accounts (As of February 2014[update]). The semiannual Drupal conference alternates between North America and Europe. Attendance at DrupalCon grew from 500 at Szeged in August 2008 to over 3,700 people at Austin, Texas in June 2014.
Smaller events, known as "Drupal Camps" or DrupalCamp, occur throughout the year all over the world. The annual Florida DrupalCamp brings users together Coding for a Cause for the benefit of nonprofit organizations.
There are a number of active Drupal forums, mailing lists and discussion groups. Drupal also maintains several IRC channels on the Freenode network.
There are over 30 national communities around drupal.org offering language-specific support.
Drupal's policy is to announce the nature of each security vulnerability once the fix is released.
Administrators of Drupal sites are automatically notified of these new releases via the Update Status module (Drupal 6) or via the Update Manager (Drupal 7). Drupal maintains a security announcement mailing list, a history of all security advisories, a security team home page, and an RSS feed with the most recent security advisories. In 2008, eleven security vulnerabilities were reported and fixed in the Drupal core. Security holes were also found and fixed in 64 of the 2243 user-contributed modules.
In a controversial article about the adoption of Drupal by the Whitehouse.gov site, Slate associate editor Chris Wilson lists some common criticisms of Drupal. Other criticisms have included:
Usability: Aspects of the Drupal 6 administration interface were seen to be confusing and intimidating to some, particularly for new administrators. According to Dries Buytaert, Drupal 7 addressed 90% of the problems identified by Usability tests conducted at the Universities of Minnesota and Baltimore. To achieve this, Acquia (the company founded by the project lead of Drupal) hired user experience designer Mark Boulton to work with the Drupal community to design an improved user interface for Drupal's administration interface. The majority of his team's design work has been implemented by the community in Drupal 7. The 2011 usability test results from the University of Minnesota Office of Information Technology show that all of the major usability problems identified in Drupal 6 are either vastly improved or non-existent in Drupal 7. However, some new usability problems were identified. Since the release of Drupal 7 there are now various distributions and applications to enhance the Back-end Usability of Drupal such as Drupal Gardens, Open Enterprise  and Mitkom Builder.
Learning curve: Some users describe Drupal as being difficult to master. Drupal's many contributed modules can have overlapping functionality and have been reported as overwhelming to new users.
Backward compatibility (for software development): Drupal does not commit to backward compatibility across major revisions. This means that module and theme developers may have to rework their code to be compatible. However, Drupal's policy is to not change how it uses one's data. This means that data from previous versions will still be usable without alteration in the new release. Drupal documents any incompatibilities, allowing the user to make informed decisions about when and whether to upgrade.
Performance/scalability: In 2008, performance tests between Drupal 6.1 and Joomla 1.5 demonstrated that Drupal's pages were delivered "significantly faster" than those of Joomla. Despite this, arguments over speed persist. Drupal is likely to be slower than a special-purpose application for a given task. For example, WordPress typically outperforms Drupal as a single-user blogging tool. Drupal positions itself for broader applications requirements that are outside the scope of more narrowly focused applications. Drupal offers caching to store various page elements, the use of which resulted in a 508% improvement in one benchmark. When using Drupal's default Page Cache mechanism, the cached pages are delivered only to anonymous users, so contributed modules must be installed to allow caching content for logged in users. Like performance, scalability (the ability to add servers to handle growing numbers of visitors with consistent response) can become a concern on large, interactive sites. MySQL's query caching can help reduce the load on the database server caused by Drupal's high query rate. Drupal caches database schema metadata as well as elements such as blocks, forms and menus. Drupal 7 increases performance in database queries and reduces PHP code usage.
Integrability with hosting structures: Because of Drupal's demanding query requirements, Drupal-based websites can quickly become very taxing to hosts whose databases reside on a machine separate from their HTTP server. While the issue can normally be addressed by implementing aggressive caching as described above, such methods may be unimplementable in cases where the host does not offer access to PHP accelerators like XCache or APC. Drupal has plugins that facilitate similar caching without requiring special PHP extensions.
The Drupal core search is ineffective at searching content: There are contributed modules that will greatly improve the search functionality on a Drupal website, but they are not easily accessible due to a high learning curve and the difficulty users have in general of finding the right module. One of the faceted search options is Apache Solr Search Integration module, however, the module requires a dedicated server or virtual private server (VPS) to operate because Solr must run on a servlet container, e.g. Tomcat, Jetty or Resin. These requirements make it harder for a Drupal website to have a functional search feature. In response some companies have created Apache Solr SaaS products.