The spiders are "bots" sent out by search engines to index pages on the web. You can see more details on the Who is online page, which shows what the various visitors are reading. At the bottom it gives details of the search spiders (Google, Yahoo, Bing, etc.)

If you don't want what you are reading to show up you can choose to be "invisible" when you log in; I've never seen any point, personally, but obviously some people are more self-concious. "Guests" are visitors who haven't logged in.