Hair emerges from hair follicles, at around 100,000 to 150,000 on every head. The hair life cycle includes three phases:

The anagen phase: this is the growth phase. It lasts for 3 to 6 years. Hair grows.
The catagen phase: this is the resting phase. Hair stops growing. This lasts for 3 weeks.
The telogen phase: the falling out phase. The hair falls out and is replaced by a new hair in the anagen phase. This lasts 3 months.