Ok Got it !The Axiom series is the "premium" line, they have a nicer keybed and some extra features (like HyperControl, a sort of automapping protocol; big display; ProTools Express included) that the others don't.
The Oxygen series are the baseline models.
The Keystations are the barebones models without faders, pads and all that - they just have the keys and the pitch/mod wheels, and that's it.