Anagen
The hair's life-cycle is made up of three phases: anagen, catagen and telogen. Anagen is the first part of this cycle and it is during this period that human hair grows. Human hair grows from small organs that are called hair follicles. During anagen, small cells at the bottom of the hair follicle, called matrix cells, begin to produce the material that makes up the hair. As more and more of the hair material is made, the hair gets pushed outward, causing it to "grow." As the hair progresses up out of the skin, it is chemically modified in a process called keratinization, which helps give it strength and also makes it water resistant. Eventually the hair is pushed up beyond the outer layer of skin, where it is visible. During this time, it is very difficult for hair to come out.
Catagen
Catagen is a transitional phase in hair growth. During this time, the hair stops actively growing and instead becomes fully keratinized. The hair also becomes thicker and the base of the hair follicle (known as the bulb) begins to degenerate as the hair follicle prepares to go in a resting state. During this time the hair follicle organ begins to shorten.
Telogen
Telogen is the "resting" state of the hair follicle. The hair follicle completely stops producing hair and the actual hair is moved up to the upper parts of the skin. The hair at this point is not as well anchored as during anagen or catagen. At this point, the hair can easily come out from just a slight amount of tension. Most hair follicles have independent cycles of anagen, catagen and telogen, so that all hair follicles are not synchronized. When a group of hair follicles enter telogen all at once, it results in large clumps of hair falling out.
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