Bots are programs that can interact with people and the blockchain. They can vote, for example. Aggroed runs a bot called @lovejuice that will upvote your posts or comments for a fee. For more information, check @lovejuice or @aggroed. There are many other bots that will upvote you for a fee, also.
Other bots, like @cheetah, @spaminator, and more, seek out and comment, or even blacklist, spammers, scammers, and plagiarists.
Hope that was some little help to you.