Those of us who live with cats know the distinctive sound of a cat vomiting up a hair ball. Cats can actually vomit voluntarily. They can choose when and where they are going to vomit. (This is why it is always on the good furniture.) It is a very useful thing to be able to do. It helps cats get rid of things they may regret eating and allows them to bring up the hair they get from grooming. Sometimes however fur balls can be nasty.
Catís tongues are specially designed for keeping the cats fur clean. They have lots of tiny barbs on them like a comb. All the barbs point backwards so any hair groomed from the coat will be swallowed. Mostly this is passed through the digestive system but clumps of hair will be vomited back up. Cats that are longhaired or recently arrived from down south lose a lot of hair so the hair balls can be quite large.
Sometimes there can be so much hair coming off the cat they can block up the intestine and cause constipation. More rarely they can even cause a cat to choke on a fur ball.
Regular thorough combing (as opposed to brushing) removes most of the loose hair especially from longhaired cats. This prevents the cat from having to swallow it. White paraffin oil or cat laxatives used regularly will help break down the hair balls and allow the hair to pass more easily. There are also special diets for cats that have particular problems with fur balls.
Fur balls are a normal part of being a cat. However, if they occur too frequently or if anything other than a fur ball is brought up then it would be best to get your cat checked.