QT MP4 Sorenson Real Aarg!

I've been struggling to truly understand codecs and such. I understand
there are 3 main players: Quicktime, Real and Windows Media. But you
can do Sorenson compression on Quicktime AND Real (I think). So I
don't really understand the relation between those formats/codecs/…
Does anyone have a good link that explains these things?

Thanks!
Peter