Hair growth has three phases: anagen, catagen and telogen. At any one time, approximately 85% of scalp hair is in the anagen phase, about 3% is in the catagen phase, about 12% is in the telogen phase.
Anagen phase is the hair growth period - it lasts about two to six years. During this period, the hair root cells are rapidly dividing, and new hair starts to form. Some people have very long hair, because their hair growth phase is longer.
Catagen phase is a transitional period, and it normally lasts for about two weeks. During this period, the hair follicle starts to shrink, and the hair stops growing. Telogen is the resting phase, and it normally lasts for about six weeks. During this time the hair stops growing, and some hair start to shed.