Are you asking about the spider in the forum listing-"Who's Online"?
If so - Spiders are web crawlers for different search engines.
They go through everything that has been posted in the internet and catalogues them.
Then someone does a search for certain word (names, location...) they have them to show.
The spiders can be Google, Bing, Metacrawler, etc...
You can check the spiders that crawl GB by going to the Who's online page; scroll to the bottom and see. We have spiders from all over the world - Asia, Europe, SA, NA...