Mood is an internal mild depression to mild elation depending on many factors. For example emotional state of an individual. A mood disorder is characterized by an excessive swing of mood. The mood state of a normal individual fluctuates between: if she passes an examination she may be little elated or happy; if she fails she may be a little depressed or sad. There is also a period. It is only when the mood swing is excessive in severity and in duration and when it interferes with a person's day-to-day activities that it becomes a mood disorder.