Think of "spiders" as the Roomba vacuum cleaners of the web--i.e. automated software that crawl around every nook and cranny of websites 24/7, sucking up text and images as they go.

That's how these search engines can catalog all those millions and millions of webpages in their database. I'm not sure what percentage of all available websites are indexed by search engines now, but thanks to spiders, it must be quite high.