In a Nutshell - it's a TCG like Magic or Yugioh, but without Boosterpacks! So you build decks and play against each others but get all the cards you need for it right out of the box! (A Living Card Game!)
Besides that, it's one of the first competitive card games with Quests in it! So you do have PVE Elements in it which can give you a Boost if you succeed at them!
There's another PVE-Quirk, the Corruption Mechanic! If the Corruption rises too high over the course of the game you and your enemy will both lose, so you will have to band together to keep it in check so you can continue fighting!
Also instead of having a fixed Turn Process you can choose your actions dynamically and have your own playstyle! Don't need to draw a Card right now? Screw it, then don't and use that action elsewhere!
—description from the designer