All of us have changes in mood. We may feel happy one day and sad the next; this is quite normal and does not mean that we depression or any other mood disorder for that matter. This is only the case when the mood is disturbed for an extended period of time.
Mood disorders are also known as affective disorders and include conditions where mood is elevated, such as mania, or where mood is lowered, such as major depressive disorder (otherwise known as clinical depression). Another mood disorder is bipolar disorder; this was previously known as manic depression, in which moods can fluctuate between low levels & mania. The most common and well-known mood disorder is major depressive disorder.