04 Aug: How do search engines decide site rankings?

I may have written about this or mentioned it: folk finding you via search engines.  How do search engines decide which websites to place higher in their listings than others? That’s a well kept secret that lots of folk get paid lots to try to second guess. It’s the field of Search Engine Optimization (SEO). This much is known: search engines value more the sites they think are more important.  So let’s start working from there.