Guests are merely those who visit the site but aren't logged in, or someone who hasn't joined the site. Most every online forum has many guests, and fewer members, unless it is a private forum with an invited list of readers. A robot is a web "spider" program that reads websites for purposes of indexing key words and listing things on search engines. If you've ever done a search on Google for a term and had Tug show up with a thread posted earlier that same day, the robots are the reason. So it's kind of a necessary thing to keep the site viable. They're harmless, and do a good service, overall.
Dave