I believe they are simply smaller LED's that can be used to make images and whatever by changing colours, etc, and since they are LED's (Light Emitting Diodes), they carry their own light.
I believe at the moment, the quality isn't as good as a standard LCD screen that tends to have a better resoloution. Am I wrong saying this? I haven't actually seen one in real, but from pictures, images don't look as clear, and slightly dotty.