A bot is an application that traverses the web in search for its goal, this goal can be anything from indexing web pages for a search engine to harvesting email addresses for spammers. There are many different kids of bots also know as spiders. Some are harmless or even very helpfull and some you just don't want on your site. Keeping away the harmless bots can be as easy as adding a robots.txt file to the root of you site. Keeping away the bad bots can be very hard to nigh impossible, all you can do is protect your site properly using a capcha for signups.