Bots

Short for robot, describes a computer program which attempts to automatically download a web site, or to perform specific functions on a web site automatically, such as signing up for an account.

Bots are associated heavily with Search Engine Spiders, and, ironically Spammers, who use Bots to automatically create accounts on free email sites for the purposes of spamming.

The most sure-fire way of preventing a bot from accessing your site is to use a CAPTCHA.