ROS provides standard operating system services such as hardware abstraction, low-level device control, implementation of commonly used functionality, message-passing between processes, and package management. It is based on a graph architecture where processing takes place in nodes that may receive, post and multiplex sensor, control, state, planning, actuator and other messages. The library is geared toward a Unix-like system (Ubuntu Linux is listed as 'supported' while other variants such as Fedora and Mac OS X are considered 'experimental').
ROS has two basic "sides": The operating system side ros as described above and ros-pkg, a suite of user contributed packages (organized into sets called metapackages) that implement functionality such as simultaneous localization and mapping, planning, perception, simulation etc.
Despite the importance of robots' reactivity, ROS is not a realtime OS, though it is possible to integrate ROS with realtime code.
ROS is released under the terms of the BSD license, and is open source software. It is free for commercial and research use. The ros-pkg contributed packages are licensed under a variety of open source licenses.
In August 2013, a blog posting announced that Willow Garage would be absorbed by another company started by its' founder, Suitable Technologies; no announcement as to the fate of continued ROS development had been made by November 2013.
HERB developed at CMU in Intel's personal robotics program
Husky A200 robot developed (and integrated into ROS) by Clearpath Robotics
PR1 personal robot developed in Ken Salisbury's lab at Stanford
PR2 personal robot being developed at Willow Garage
rosbridge protocol and server Brown University developed the rosbridge protocol to enable any robot or computing environment to integrate with ROS using JSON-based messaging, such as for common web browsers, Matlab, Microsoft Windows, OS X, and embedded systems
STAIR I and II robots developed in Andrew Ng's lab at Stanford
SummitXL: Mobile robot developed by Robotnik, an engineering company specialized in mobile robots, robotic arms and industrial solutions with ROS architecture.