In Sanctum, you are a mighty mage, with a nation of minions to control. Your spells are the cards in your deck, and those spells cost Mana. To play the game, you first have to register, and then download the Sanctum client. When you register you receive a 60 card starter deck which is usually enough cards to get a decent deck together. A lot of players choose not to purchase cards, and you will see a lot of people saying they will only play others with Starter Decks only.
Sanctum is primarily a game of strategy and luck.
It is played by two players, real time over the Internet. The premise of the game is basically to get at least one of your minions into your opponent's Sanctum, or force him to Concede. The strategy component of the game starts when you build your deck and carries over into general play. The card you get can make the difference in a game. Bloodlines added 60 cards to the game, but it also added a ton of new features, terrians, and strategies to the game.