A bot is a program that runs on a computer 24/7, automating mundane tasks for the owner, even if the owner is not logged in. Bots are used on the Internet in a variety of ways, most popularly in IRCs (Internet Relay Chat) and in search engines.
IRC bots are programs that connect to an IRC network and interact with IRC in very much the same way a normal user does (in fact, IRC servers treat bots as regular users). Most IRC bots are used for channel control. Also known as "automatons," bots are disliked by IRC operators and long-time IRC users because of the system resources they consume.