Taken from Wikipedia:
Trichotillomania (TTM), also known as hair pulling disorder, is a mental disorder characterised by a long term urge that results in the pulling out of one’s hair .  This occurs to such a degree that hair loss can be seen. Efforts to stop pulling hair typically fail. Hair removal may occur anywhere; however, the head and around the eyes are most common. The hair pulling is to such a degree that it results in distress.
The disorder may run in families. It occurs more commonly in those with obsessive compulsive disorder . Episodes of pulling may be triggered by anxiety . People usually acknowledge that they pull their hair. On examination broken hairs may be seen. Other conditions that may present similarly include body dysmorphic disorder , however in that condition people remove hair to try to improve what they see as a problem in how they look.