From the Fantasy Flight games website:
The Call of Cthulhu: The Card Game is a game of horror, insanity and terror for two. Take on the role of intrepid investigator trying to solve the mysteries of the ancient ones, or a servant of the Mythos and carry out vile conspiracies, or any combination of the two.
The Core Set is the best way to start playing it contains a game board, high-quality pieces and cards for seven factions that can be mixed and matched into a variety of different decks, right out of the box.
By buying Asylum Packs, fixed 40 card supplements that feature 20 distinct cards, players will be able to customize their core decks, or create strategies and decks of their own.
Starting with the Yuggoth Contract, each Asylum Pack will now feature 3 copies each of twenty unique cards.