Depression is the most common of the disorders, with one in five people experiencing bouts each year.
It can happen in short periods of minor upset or for longer periods; and there are a multitude of possible causes.
Those who suffer from depression may lose interest with friends and family, feel hopeless or find difficulty concentrating.
Although depression can be a normal aspect of life, for many it can be a destructive condition, with some even resorting to suicide.
The likelihood of developing the illness is increased if someone has a family history of depression. It can also be caused by events such as losing a job, the end of a relationship, death of a loved one, substance abuse and loneliness.