What's the difference between regular depression and being manic depressive?

Mania: Regular (or "unipolar") depression just has low moods, not abnormally high ones. In contrast, manic depression, now called bipolar disorder, has low moods as well as mania, or abnormally high moods. These are like being under the influence of a strong stimulant: rapid speech, excitement, poor social judgment, irritability, and often grandiosity.

Mania itself: People who suffer from manic depression ("bipolar") have two poles of extreme mood--both depression (low) and mania (high) as colleagues have described. It's important to realize that with just episodes of depression, it is not certain whether the depression is "unipolar" or part of "bipolar". Once one has a manic episode, bipolar is confirmed, even if the depression has not yet appeared.