As much as it is true that hair is not composed of living biomaterial, it still certainly has a ‘life cycle’. Here are the different phases in the life cycle of a hair strand and the highlights of each, in a nutshell. 

The first and longest lasting phase is the Anagen phase. This is the growth phase in which the hair shaft is actively growing in length. Healthy hair grows a little over a centimeter in length each month on average. The anagen phase typically lasts a couple of years and varies from person to person and with age for the same person. For some people, this phase can last up to 10 years giving them the ability to grow their hair out long. A long anagen phase just means that the hair is able to grow for longer periods of time before it is shed. Most of the hair on a scalp is in the anagen phase at any given time. In the image, the Anagen phase shows a hair strand connected to the dermal papillae which supports its growth. 

The Catagen phase is the short, in-between phase that occurs after the Anagen phase is complete and before the next phase starts. It is a transitory phase and only lasts several days. Only a small fraction of the hair on the scalp is in this phase at any time instance. The defining activity in the catagen phase is that the hair shaft decouples from the hair bulb. The hair strand is unable to grow anymore since it does not have access to the blood and nerve supply that the bulb provides. The strand in Catagen in the image is in the process of dissociating from the growth mechanism.

The Telogen phase is a resting phase which happens to also be the last phase before a hair strand is shed. About 10% of the hair on a scalp is in the telogen phase at a given time. A hair strand going through this phase typically continues to stay in place dormant until new hair starting its anagen phase pushes it out. The telogen phase typically lasts a few months. The above image shows a hair strand in an unconnected state which represents Telogen.  

Although not always recognized as such, for completeness, there is a fourth phase in the hair growth cycle called the Exogen phase. This is merely the shedding phase where the hair strand falls out. Exogen would occur right at the end of the Telogen phase. It is normal to have anywhere from 50 to 100 strands a day going through Exogen and not be considered hair loss. 

The phases described above are largely true for terminal hair. Terminal hair is the long, thick and pigmented type that is generally found on the scalp, arms, legs and certain other bodily areas. Most conversations about hair styling, treatments and products are largely in the context of terminal hair. In contrast, vellus hair is fine, unpigmented and hardly visible. These hairs are generally deprived of the medulla layer (innermost or core part of hair) and grow on most skin surfaces other than palms and soles. 

As with most things, the phases of hair described here can vary in quality and duration based on age, hormones, overall health, nutrition and hair care habits.