IGN provides us with the rules of the card game invented for Fallout: New Vegas, called Caravan, which can be played in real life and as a mini-game within the title.
Caravan plays a bit like a head-to-head version of Blackjack, with the objective being to build sets of cards -- or Caravans -- with combined values between 21 and 26. You'll need at least two decks of cards to play, as each player sets up their own deck with any 30 cards. It's fine to have the same cards as your opponent, but you yourself can't have more than one of any specific card. In other words, your deck of 30 can't have two ace of spades.
The rules are pretty simple, but there are a few things to keep in mind as you work to build your three caravans filled with cards of the same suit. Rather than paraphrase them and confuse anybody, I'll list them verbatim.