Bipolar disorder
Bipolar disorder was previously diagnosed as ‘manic depression’. As the older name suggests, someone with bipolar disorder will have severe mood swings. These usually last several weeks or months and are far beyond the experiences of the general population. The symptoms can be described as follows:
Low: 'Depressive' feelings of intense depression and despair.
High: 'Manic' feelings of extreme happiness and despair.
Mixed: Depressed mood with the restlessness and overactivity of a manic episode.
About 1 in every 100 adults has bipolar disorder at some point in their life. It usually starts during or after the teenage years. It is unusual for it to start after the age of 40. Men and women are affected equally.
Bipolar I
To meet the diagnostic criteria for Bipolar I, the individual must have experienced at least one high, or manic episode, that has lasted for longer than one week. Some people with Bipolar I will have only manic episodes, although most will also have periods of depression. When untreated, manic episodes can generally last 3 to 6 months, with untreated depressive episodes lasting 6 to 12 months.
Bipolar II
To meet the diagnostic criteria for Bipolar II, the individual must have experienced one episode of severe depression, but only mild manic episodes (hypomania).
Sources
About.com: What is Bipolar Disorder?