What some of you see is called remarketing.
"By identifying those who have visited your site recently, remarking works by then showing these people your ads as they view other sites that are part of the Google Display Network."
Basically, when you view some website, such as looking to buy some shoes online. You checkout, but don't pay, changing your mind. When you visit this site, and others, where a page has something to do with clothing, you will get shown shoes, and more specifically, if the site you visited runs Google ads, then you see their shoes, the site you changed your mind upon. This kicks your brain into gear to maybe go and buy those shoes after all, thus you return to their website.
Ads are shown based on your browsing history, searches, sites visited, country, state and town, and lots more. Its how both Google and sites like this make their money, by putting relevant ads in front of the individual.
Agreed though... there are some crackers I come across at times here.