Depression is recognised as an illness, but there are still arguments about exactly how it is diagnosed and what the types are. Basically, depression is not just being unhappy. Symptoms include loss of energy, feelings of worthlessness or hopelessness, and often physical changes like loss of appetite and inability to sleep. There are several ways to classify depression.

Health professionals talk about primary depression, which has no apparent cause, and secondary depression which may be triggered by a disturbing life event or physical change.

Another distinction is between unipolar and bipolar depression - the bipolar form involves great changes of mood, often between excitement and despair, and the unipolar form doesn't have the 'up' phases.

There is also a difference between chronic mild depression, which may go on for years and not respond well to treatment, and a major depressive illness.

You can learn more about types, symptoms and treatment of depression here.