How Internet Search Engines Work is a nicely-done, quick overview of how crawler-based search engines (aka "spiders") work. Marshall Brain has done a great job putting together a really useful and fun site about "How Stuff Works". If you've never visited this site before, be sure to jump to the home page after you finish reviewing how search engines work--you'll find everything from "How Facial Recognition Systems Work" to "How Fat Cells Work"!