Comparing SD Satellite receivers to HD Satellite receivers. What's the difference?

I know HD is way better then SD but are there differences in the receivers that make that happen. I know when sending SD signals via satellite the signals are compressed and will give you a not so clear image especially when viewing on a HDTV like I am. So does a HD receiver uncompress those signals and if so how, is there another special component in the HD receiver that is not in a SD receiver, like a box that uncompresses the signal. So pretty much what is the difference between a SD receiver and HD receiver?