Gwent: The Witcher Card Game, as you might expect from the name alone, is a standalone version of The Witcher 3's endlessly addictive minigame. It features all of the same strategies and intrigue, but with actual balance, less overpowered dragons, and a whole bunch of new cards to toy around with.
If you haven't given it a try, now would be a good time to sign up for the Closed Beta as Gwent has just recently been updated with not only the new Nilfgaard faction, but also a bunch of extra cards as well! As for what exactly Nilfgaard brings to the table, it shouldn't come as much of a surprise to hear that they are focused entirely around control, treachery, and using the enemy's strength against them. If you want to see all of this in video form, but also get a brief look at all of the other factions, here's the most recent "Factions in Gwent" trailer:
For those of you interested in my thoughts on Gwent, you can find my review of the initial beta version by heading over here. However, if you're just interested in the short version, here it is: "Gwent might currently be in an early closed beta state, but even now it is a vastly superior version of The Witcher 3's minigame. The balance is tighter, there are more viable cards with which you can craft decks, and the card abilities themselves have been greatly redesigned to offer a much more strategic experience. If your only wish from Gwent: The Witcher Card Game was to get a new and improved version of The Witcher 3's Gwent - you can consider your wish granted!"