Some time ago CSS "frameworks" were growing out of the ground like mushrooms after rain. Now we have at least a dozen, and the number is growing.
I have always wondered what similarities/differences they have, who knows, i might use one of them in the future. The answer comes from Eric Meyer, who has made some comparisons recently, which was summed up Jeremy Keith in this article. A good reading if you have two minutes.
Some other info on the topic can be found here and here.