ishinglifin,
Bots are basically web robots that run automated tasks over the net at a fast rate. There are thousands of different types and new ones being written every day. They are essentially a program that runs one or thousands of tasks. As BurningChrome said Bots can cover the spectrum, everything from Google indexing for their search engines so you can find things on the web to malicious ones that plant ads, try to insert malicious code or even like Chatbots that answer posts on twitter and Facebook to enforce or change points of view. Bots are even written to help you do a task online, like help lines.