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 losscan 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.