I just clicked on Members and then Current Visitors, makes interesting reading, Members, Guests and Robots (don't fully understand Robot, just a vague idea).
Going like glue at the moment. About 30 seconds to a minute between clicking on "Reply" and getting the quote in this text area.
With regards to Robots - there are sites like Google who have automated programs (web crawlers) which spend all their time following links from web site to web site and adding information to their database.
These are usually identified by the fora and shown as "bots" or similar on the statistics.
There is even a special file "robots.txt" which can be used by sites to ask the robots not to search and index part (or all) of the site.
Not all web crawlers respect the request, unfortunately.
Anyway, the robot web crawlers are why you can use Google (or other search engine) to find discussion threads on this site.