I think I figured out part of Google's search method.

User searches for term in Google,
Google pulls up results by keyword (crawling), linking algorithms, and relevance.

User clicks on a result on the second page.

Google records that click, and bumps the relevance of that page.

I noticed this when searching for a really obscure term (my wow guild name) in google. The first time I searched for it, it appeared on the second page. The following times I searched for it, it got bumped up until it was the first result.