They're software applications that search the web for information. Google for example has bots that enter sites to look for data that can improve its searches. Called data mining. Company bots can search sites to see if people are talking about a certain product, or search for email addresses, or whatever they're interested in.
Bots listed on a site have told the site that they are a bot and the site's software can negotiate terms. For example, Google could send 1000000 bots into a site all at once and crash the site. It's the same as if 1000000 people all signed into a site at once and overloaded the site. So bots register to find out how to act properly. This register thing isn't like a user name and password, it's just the bot saying "hi, I'm a bot. Is it OK if I read 1 page per minute?".
Then there are bots that don't tell a site they're a bot so the site assumes they are a person. Those bots will appear at "guests".
Then there are bots that act like a member, with a name, password. They can make posts. They read lots of posts to see how to talk to people.
Anyone can create a bot and let it roam the web. They're very easy to make.