More or less Wine + some experimental patches not yet I twgrated in mainstream wine + a buch of DirectX translation libraries + close steam integration.
I've heard it's pretty good for fixing video playback/rendering (e.g. cutscene) issues if both the stable and the experimental branch of Proton can't make it work.
So aside from the stuff that has been implemented differently, running Proton instead of Proton GE is like trying to game on Windows N editions.
I absolutely love my Ally running SteamOS. Incredible work by... everyone involved, really.