A bot is common parlance on the Internet for an automated web agent. It is derived from the word "robot", reflecting the autonomous character in the \'virtual robot'-ness of the concept. Bots are typically employed to gather information, when they are usually referred to as spiderss, such as that used by e.g. Google to gather web-page information, or they may be used to interact dynamically with a site in a particular way, for example to exploit or locate arbitrage opportunities for financial gain.
The term bot also frequently refers to an interactive agent/program that communicates with users, for example a program that communicates with people via IM, IRC, or web interface. These may allow people to ask questions in plain english and then formulate a proper response. These bots can often handle many tasks including reporting weather, zip code information, sports scores, converting currency or other units, and much more. An additional role of IRC-bots may be to lurk in the background of a conversation channel, commenting on certain phrases uttered by the participants (based on pattern matching). This is sometimes used as a help service for new users, or even for mild censorship (e.g., bad language).
There has been a great deal of controversy about the use of bots in an automated trading function. eBay has been to court in an attempt to suppress a third party company from using bots to traverse their site looking for bargains; this approach backfired on eBay and attracted the attentions of further bots. The UK based bet exchange Betfair believe they have also experienced some difficulties with bots on their site but are taking a more "hands off" approach in the light of the eBay experience.
The term bot is used frequently in videogame, referring to computer controlled enemies in a multiplayer game, that manage to recreate the actions of a human player. These bots are used in training, before playing over Internet, or simply to maximise the experience when there aren't enough players in the game.
See Also: Chatterbot