Hair growth follows a cycle with three phases.
The first phase is called anagen and this is the active growth stage. During anagen a hair shaft (the technical term for a strand of hair) is created from the dermal papilla cells (the dermal papilla cells are just below the hair follicle in the skin). The hair shaft becomes covered with the protein keratin and extends from beyond the pore of the skin and keeps growing from the base of the follicle. The hair shaft also extends deeper into the skin to get nourishment from blood vessels allowing for growth. This first phase can last 2-8 years. Most of our hair (around 80%) is in the anagen phase.

The second phase of hair growth is called catagen and this is referred to as a transitional phase since it only lasts a couple of weeks. During catagen the hair shaft breaks away from the blood vessels and stops growing (but it doesn't fall out).

The final phase of hair growth is called telogen. This phase lasts for 2-4 months and is referred to as the resting phase. It is during this phase or the start of the next anagen phase when the hair shaft falls out and a new shaft begins.