"Net bots" or "bots" for short are software applications that run automated tasks over the Net. As Prox says, a lot of the spam is generated by bots.
Google also uses bots, which scan the Net and feed information back to the Google databases which is then used when you do a search.
The Google Feedfetcher is a bot which searches for RSS or Atom feeds (which are something generated by some websites, such as blogs), and uses them for Google's blog search at
http://blogsearch.google.com/ .
Google[Bot] is another one that you may see from time-to-time. From Wiki; "Googlebot is the search bot software used by Google, which collects documents from the web to build a searchable index for the Google search engine".
It does *not* retrieve specific information (member's user-names, the details of posts, etc) which is obviously an important security/safety issue. For example, when I Google "one out of none" (try it

), nothing relevant to you is retrieved.