Over at SEOmoz there is a great page that explains how search engines go about ranking your pages.
This article contains a large list of the factors that can
influence a web document's rank at the major search engines (Yahoo!,
MSN, Google & AskJeeves) for a particular term or phrase.
This is great stuff for any Web publisher to know. Search engines
can be a bit mysterious with their rankings at best, but knowing even a
little can help you set yourself up to be found.