Bot is short for Robot. A bot is a program written by a user that acts like and may appear to be an actual user, depending on the skill of the programmer. Bots are not looked on favorably by most Networks. They are even banned by many servers. Bots have been given a bad rap due to misuse and abuse by hackers.
Bots can, however, be very useful if properly programmed.
They can be user to hold a channel open when the owner is not physically there on the channel, they can be set up as a file server to offer files to people, they can be uses as help and information servers, or they can even be used to run games on a channel. Bots can range from the very simple to the very complex. The most powerful Bots are run on ircll client programs, which is a UNIX based system. Bots are not for everyone, and I would advise you not even consider using one. A Netsplit is the loss of communication between servers on a Network.