Major Depressive Disorder (MDD): A mood disorder characterized by severe symptoms that interfere with one's ability to work, sleep, study eat and enjoy life.
Dysthymia (Persistent Depressive Disorder): A chronic form depression where individuals have less severe but long-term depressive symptoms that last for at least two years.
Seasonal Affective Disorder (SAD): This is depression associated with late autumn and winter and thought to be caused by a lack of light.