How does Google decide which search results to show first?

This is the great question of SEO. There are over 200 factors that google uses to determine which site is the most relevant for any giving search term which include the number of links pointing to that site, the quality of those links, how much fresh content is on the site, etc. google is always changing these factors and only releases a few to the public, usually the ones that matter most (or so they tell us).