A handshake is a short ritual in which two people grasp one of each other's like hands, in most cases accompanied by a brief up and down movement of the grasped hands. When making a bet and confirming the bet with a handshake; the instant that the two hands touch willingly, the bet becomes official.
The first handshake on human history is documented by the monument of Kalhu, showing the Assyrian king Shalmaneser III and Marduk-zakir-šumi I of Babylon shaking hands in a public display of Assyro-Babylonian friendship.  Archaeological ruins and ancient texts show that handshaking was practiced in ancient Greece as far back as the 5th century BC; a depiction of two soldiers shaking hands can be found on part of a 5th-century BC funerary stele on display in the Pergamon Museum, Berlin (stele SK1708) and other funerary steles like the one of the 4th century BC which depicts Thraseas and his wife Euandria handshaking (see images on the right). The handshake is thought by some to have originated as a gesture of peace by demonstrating that the hand holds no weapon.
Pictures: Handshaking depicted on historic artifacts
Leaders welcome a boy into Scouting, March 2010, Mexico City, Mexico. Note the left-handed handshake.
There are various customs surrounding handshakes, both generically and specific to certain cultures:
The handshake is commonly done upon meeting, greeting, parting, offering congratulations, expressing gratitude, or completing an agreement. In sports or other competitive activities, it is also done as a sign of good sportsmanship. Its purpose is to convey trust, respect, balance, and equality. If it is done to form an agreement, the agreement is not official until the hands are parted 
Unless health issues or local customs dictate otherwise, usually a handshake is made with bare hands. However, it depends on the situation.
In Anglophone countries, in business situations. In casual non-business situations, men are more likely to shake hands than women.
In Belgium, handshakes are done more often, especially on meetings.
In Switzerland, it may be expected to shake the women's hands first.
Austrians shake hands when meeting, often including with children.
In Russia, a handshake is rarely performed by opposite sexes. Man shaking hands with women can be considered impolite, since hand-kissing is preferred as a ritual for greeting a lady. However, kissing the hand is considered unsuitable for business situations.
In some countries such as Turkey or the Arabic-speaking Middle East, handshakes are not as firm as in North America and Europe. Consequently, a grip which is too firm will be considered as rude. Hand shaking between men and women is not encouraged in countries where the majority religion is Islam.
Moroccans also give one kiss on each cheek (to corresponding genders) together with the handshake. Also, in some countries, a variation exists where instead of kisses, after the handshake the palm is placed unto the heart.
In China, where a weak handshake is also preferred, people shaking hands will often hold on to each other's hands for an extended period after the initial handshake.
In Japan, it is appropriate to let the Japanese initiate the handshake, and a weak handshake is preferred.
In India and several nearby countries, the respectful Namaste gesture, sometimes combined with a slight bow, is used in place of handshakes.
In Norway, where a firm handshake is preferred, people will most often shake hands when agreeing on deals, both in private and business relations.
In South Korea, a senior person will initiate a handshake, where it is preferred to be weak. It is a sign of respect to grasp the right arm with the left hand when shaking hands. It is also considered rude or disrespectful to have your free hand in your pocket while shaking hands.
Related to a handshake but more casual, some people prefer a fist bump. Typically the fist bump is done with a clenched hand. Only the knuckles of the hand are typically touched to the knuckles of the other persons hand. Like a hand shake the fist bump may be used to acknowledge a relationship with another person. However, unlike the formality of a hand shake, the fist bump is typically not used to seal a business deal or in formal business settings.
The Hand Hug is a type of handshake popular with politicians, as it can present them as being warm, friendly, trustworthy and honest. This type of handshake involves covering the clenched hands with the remaining free hand, creating a sort of "cocoon."
Scouts will shake hands with their left hand as a gesture of trust. This originated when the founder of the movement, Lord Baden-Powell of Gillwell then a British cavalry officer, met an African tribesman http://en.wikipedia.org/wiki/Scout_handshake.
On Memorial Day 2008, two friends from Muscatine, Iowa, Kevin Whittaker and Cory Jens set the Guinness World Record for the world's longest handshake at 9 hours and 30 minutes in San Francisco, CA. On 21 September 2009, Jack Tsonis and Lindsay Morrison broke the Guinness World Record for the world's longest handshake, shaking hands for 12 hours, 34 minutes and 56 seconds. This record was broken less than a month later in Claremont, California, when John-Clark Levin and George Posner shook hands for 15 hours, 15 minutes, and 15 seconds. The next month, on 21 November, Matthew Rosen and Joe Ackerman surpassed this feat, with a new world record time of 15 hours, 30 minutes and 45 seconds., certified in the latest edition of the Guinness Book of Records on page 111. At 8pm EST on Friday 14 January 2011 the latest attempt at the longest hand-shake commenced in New York Times Square and the existing record was smashed  by semi-professional world record-breaker Alastair Galpin and Don Purdon from New Zealand and Nepalese brothers Rohit and Santosh Timilsina who agreed to share the new record after 33 hours and 3 minutes.
^Busterson, Philip A. Social Rituals of the British.
^"Dear Uncle Ezra - Questions for Tuesday, April 3, 2007". Cornell University. 3 April 2007. Question 8. Retrieved 4 September 2011. "There are many conflicting theories about the origin of the handshake. It seems that the most common one involves the evidence of the lack of a weapon in the right hand, which normally bears a weapon. It is shown to be empty by its connectedness to the opposite person's hand."[broken citation]