via Makezine:
What is Yahoo! Site Explorer?
The Yahoo! search database contains detailed information about the structure of the web. In addition to the web pages themselves, the database stores information about links among pages, and uses that information (as well as additional algorithms) to gauge the popularity of a given page.
Site Explorer gives you access to this information so you can learn about a site. To explore a site, you submit a URL using a search box, just as you would for a normal web search. You can then click links on the results page to see detailed information (source)
And the search war continues.