If you look at News and Announcements » The forum software has been upgraded Krystal gave us a very clear explaination!!

Spiders are sent out by the search companies like Google. They crawl all over the Web indexing the content of sites for their search engines.

Oh thanks Arnie! Explaining the outside source, like Google, helped. I thought maybe it was an internal program for the forum searches I know, I'm slow I know it's off-topic a bit, but is this how certain web pages come up first in searches? I always thought the result that came first was determined by fees or hits on the site.