Steam games vs CD Keys games, what's the difference?

Posted 5th Oct 2015
I'm making the move from console to PC soon and after checking out the games catalogues online I noticed that CD keys sell games much cheaper than Steam but what is the difference between buying from either site (South Park: SoT without DLC is £9.99 on CD Keys and £23.99 on Steam) ? I've noticed the games on CD keys show Steam as one of their platforms so I'm assuming they can be loaded through Steam?
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