What is the difference between mental health and mental retardation?

These are two terms which hardly have anything to do with each other.
Mental retardation is usually a state in which a person is born into because of certain pre-natal events, or just genetics. It is characterized by extreme under-average intelligence, and other behavioral disorders.
Mental health refers to how healthy a person is mentally. Issues that affect mental health are abnormal psychological diseases such as schizophrenia, bi-polar disorder, and so on. Other issues that affect mental health are relatively simple issues like stress, anxiety, and so on.

PS: synergism721 said it the best. Check his answer

mental health=psychological well-being and satisfactory adjustment to society and to the ordinary demands of life.

mental retardation=a developmental disorder characterized by a subnormal ability to learn and a substantially low IQ.

Mental health relates to healthy thoughts ,feelings and knowing what is socially acceptable etc, mental retardation is where thoughts and feelings are handicapped, or limited and can not develop normally.It is not cotrollable and their is no concept of what is socially acceptable behaviour.

Are you serious?????????

"Mental health" is a general term referring to the state of mind of a person. Good mental health would include the ability to problem solve and socialize. It also involves how the person feels about themselves and the choices they make. Other popular terms related to mental health are "self esteem", "confidence", and "self worth". As I said, this is a general definition and by no means exhaustive. "Metal retardation" is a term once used to describe a condition where a person is functioning below the level that would be expected for their age. The term "developmental disability" is now more commonly used. There are many causes, some known, some unknown. The degree of impairment, or disabiliy, can vary widely. Many individuals with developmental disabilities live on there own with little or no supervision. Others require 24 hour supervision and must have all their activities of daily living taken care of by others. Hope this helps.