What is mental health?

Mental health is the state of cognitive and emotional wellbeings or an absense of mental illness.

Common mental illnesses include:
1) obessive-complusive disorder (強迫症) - a psychiatric diagnosis that describes an anxiety disorder most commonly characterized by obsessive, distressing, intrusive thoughts and related compulsions (tasks or "rituals") which attempt to neutralize the obsessions. OCD is distinguished from other types of anxiety, including the routine tension and stress that appear throughout life.

2) depression (抑鬱症) - a pervasive low mood, loss of interest in usual activities and diminished ability to experience pleasure. The diagnosis is made if a person has suffered one or more major depressive episodes

3) schizophrenia (精神分列) - bnormalities in the perception or expression of reality. It most commonly manifests as auditory hallucinations, paranoid or bizarre delusions or disorganized speech and thinking in the context of significant social or occupational dysfunction

4) anorexia nervosa (心因性厭食症) - an eating disorder characterized by low body weight and body image distortion with an obsessive fear of gaining weight. Individuals with anorexia are known to commonly control body weight through the means of voluntary starvation, purging, vomiting, excessive exercise, or other weight control measures, such as diet pills or diuretic drugs.

Details pls refer to the websites:
http://en.wikipedia.org/wiki/Mental_health
http://www.nami.org/