http://en.wikipedia.org/wiki/Bipolar_disorder

Basically, instead of feeling depressed all the time, someone who suffers from Bi Polar disorder cycles in/out of Depression and into some form of Mania, either full-blown, or mild/hypo-mania. Some also experience mixed affective episodes, which have a mix of depression and mania in one combo or another. Such as Dysphoric Mania, or Agitated Depression. But i would start with the above wiki article.

In contrast, here is the major depressive disorder page, which is more commonly known as "depression" or "clinical depression" http://en.wikipedia.org/wiki/Major_depressive_disorder