Hair grows on the human skin except for the palms and soles. Made up of a Keratin (a protein that your nails are also made up of), hair is created in hair follicles that lie on the outer layer of the skin. These follicles are responsible for production of new hair cells. As new cells are produced, the old cells are pushed out through the surface of the skin. This is how hair grows.
So, your hair is basically a string of dead keratin cells.
Each of the hair follicles have a life cycle of its own. This life cycle can be influenced by age, disease and a number of other factors and is divided into phases.
Phase 1: The Anagen Phase/Growing Phase
Duration of 2-6 years. Active hair growth occurs in this phase. This phase determines the hair length.
Phase 2: The Catagen Phase/Regression Phase
Duration of 10 days or in some cases, 2-3 weeks. Transitional hair growth occurs during this phase with the shrinking of hair follicles.
Phase 3: The Telogen Phase/Resting Phase
Duration of 2-3 months. This is a resting phase at the end of which, hair sheds and is replaced by new hair. The growing cycle starts again once the resting phase is over.
Phase 4: The Exogen Phase/Shedding Phase
In this period, the resting hair detaches from the follicle and sheds.
Note: It should be noted that the rate of hair growth slows down as we age.