As the name implies, a hairball is in fact a ball of hair. The clump of ingested fur forms in a kitty's tummy and, when it becomes too big, causes that "present" found on your carpet. If you're noticing (or worried about) this problem, switch to a high-quality, special hairball formula food or a petroleum-based digestive aid. Already have a picky kitty on your hands? Get my tip and read more.
Try adding a teaspoon of yummy canned pumpkin or baby food squash to his normal food: The sticky stuff can help coat the tummy's contents and hopefully allow your cat to smoothly pass the hairball out the other end and into the litterbox instead.