If you've seen a bunch of cats hanging out together — a fellowship of felines, if you will — you can say that you've seen a clowder, the collective term for a group of cats.
The term clowder is usually reserved for a group of domestic or feral cats, rather than for a group of wildcats. (For example, a group of lions is a pride, and a group of cheetahs is a coalition.) The word clowder is also specific to a group of adult cats; a group of kittens is usually called a litter. Other terms that can be used for a group of cats are a pounce — especially if the cats are being playful — and a glaring — if the cats are staring at each other suspiciously.