Cats groom themselves on a daily basis to remove dead hair and foreign material from the coat. The fur of both short and long-haired cats has an ongoing cycle of growth, maintenance, loss, and replacement. The growth stage, termed anagen, lasts about 2-3 months and is followed by catagen, a short period where mature hairs are alive and have achieved full length. The final stage is telogen, where the hairs gradually die and are shed over a few months. Grooming helps to clear the fur of these old hairs, and stimulates growth of the new ones. Shedding occurs at all times, and because the hairs on any particular area of the body are in different stages of the hair cycle, the fur never looks "patchy" if all is normal.
Cats have very rough tongues. The top surface of the tongue has barbs that help the cat pull old dead hairs from the fur. These hairs are swallowed. If a cat takes in too much hair, hairballs may form and the cat may vomit. The saliva wets the hairs down and helps to maintain the distribution of natural oils through the hair coat.
Outdoor cats shed most heavily in spring and fall and indoor cats shed year-round. This is because there is constant light in the home, and there are no temperature extremes, so the body is fooled into thinking that it needs to turn over the coat gradually rather than seasonally.
It is normal for a cat to spend lots of time grooming every day. They will lick their fur, nibble to remove small mats and foreign objects such as sticks and burs, and will chomp on their nails to remove the dead nail sheaths if clawing and scratching on the post has not removed them.
Excessive grooming can occur:
when a cat is stressed in response to excess shedding due to general illness or pregnancy/birth due to skin irritation such as that seen in allergic dermatitis
if chemical irritants such as soap are left in the coat after bathing in the presence of external parasites such as fleas when foreign materials such as burs, twigs or dirt are stuck in the fur in response to bacterial or fungal skin infections in response to scolding or apparent embarrassment.