Aaron Wall has posted an excellent article on search engine relevancy over at SEOBook.com explaining his perception of the different relevancy criteria used by Google, Yahoo and MSN. Aaron offers both a short version and long version. Yahoo is great, but can still be manipulated. Google is best at determining relevancy. MSN is still new and currently easiest to manipulate. Ask is good, but their small market share does not necessarily warrant specific attention from a SEO perspective.
This is a great resource offering some very useful tips and insights for those that are grappling with understanding how search engines determine what’s relevant. This article is not just for newbies though. Aaron pays attention to concepts like the importance of site age, editorial and social factors that you don’t see in other articles of this sort and that are increasingly important.