A bot (short for "robot") is a program that operates as an agent for a user or another program or simulates a human activity. On the Internet, the most ubiquitous bots are the programs, also called spiders or crawlers, that access Web sites and gather their content for search engine indexes.
A chatterbot is a program that can simulate talk with a human being. One of the first and most famous chatterbots (prior to the Web) was Eliza, a program that pretended to be a psychotherapist and answered questions with other questions.
Red and Andrette were names of two early programs that could be customized to answer questions from users seeking service for a product. Such a program is sometimes called a virtual representative or a virtual service agent.
A shopbot is a program that shops around the Web on your behalf and locates the best price for a product you're looking for. There are also bots such as OpenSesame that observe a user's patterns in navigating a Web site and customize the site for that user.
A knowbot is a program that collects knowledge for a user by automatically visiting Internet sites and gathering information that meets certain specified criteria.