This is an impulse control disorder that refers to the urge in some individuals to pull out hair from the head, eyelashes, eyebrows, nose hair and other facial hair and even pubic hair.
Stress factors in the environment, depression and internal stress could also be stress factors sometimes.
It is sometimes classified as habit behavior such as nail biting and picking of the skin, but anxiety, depression and obsessive compulsive disorder are more frequently encountered in people with Trichotillomania. 
Sufferers of this disorder can seem to live quite normal lives but could have bald patches on the head or noticeably missing or patchy facial hair.
People suffering from this disorder will often be triggered by certain particular stress factors and can additionally suffer low self esteem because of the disorder they fear ridicule and rejection due to the tell tale physical signs of the disorder.
Sometimes people with this disorder may also chew or eat the hair that they pull out. This is referred to as Trichophagia. In extreme cases this can lead to Rapunzel syndrome, and in very rare cases even death.
The problem with many who have this problem is that they think they are alone in this since very few cases of Trichotillomania are actually reported.
