They are based on google's history of your surfing (and possibly emails, etc if you use google for other services). It is the google side of things that is choosing these ads for you not the websites themselves.
"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.