Photo by kevindooley (Flickr)
Cat shedding is apparently governed mostly by changes in the ambient (surrounding area) light. Some people think it is dictated by the seasons. There is obviously an overlap as in the winter the ambient light levels are lower and in the summer the light is brighter.
The more the cat is exposed to natural light the greater the shedding. This makes sense. Cat coats are heavy and hot (potentially). We sometimes forget that. My old lady spends most days outside on a bench in the shade keeping cool. She has a double coat. My three legged boy has a single black coat and is no doubt cooler.
Outdoor cats will usually, therefore, shed more than indoor cats. In the late spring the shedding process is triggered in outdoor cats. The coat becomes thinner - a summer coat.
During late autumn (fall) the coat will thicken for the winter ahead.
If the climate is a stable, bright, hot one the cat may shed constantly and regrow constantly. Places such as the Canary Islands have these perfect climates.
Most cats have double coats (see cat hair) and the undercoat is fine and soft. The undercoat may shed in a patchy way. Personally I have not noticed this. The cat may look "moth-eaten'!
Dead hairs should ideally be removed with a grooming brush. This is pleasurable for both human guardian and cat.
Dead hairs may irritate the skin of a cat.
Cat breeds with a single, fine haired, coat, the Devon and Cornish Rexs, shed less than other breeds it is said. But all cats shed. There is no such cat as a non shedding cat breed and in any case purebred cats are relatively rare.