With the absurd processing power of computers nowadays, we are seeing the development of efficient algorithms for just about anything you can think of. Some of the more popular uses are for searching (Google), advertising (Google), and tracking web browsing activity of different users (now, too, Google).
Basically, the premise is that it tracks your browsing history and archives it for you, so you are able to search pages that you have already viewed since whenever you signed up for the service. It also uses your browsing trends to rank pages according to what you've looked at before. I find it slightly frightening that we're giving Google and their web servers information about what we do on the internet ('Now what I do, say, and view on the internet matters? Oh no!'). Amazon has a service like this as well that tracks your shopping history on their site and recommends things you might like to buy ('Buy this! Buy this!). Scary to think of the prospect that a bunch of code can know more about us than we ourselves do....