But here is the short answer. Normal mood cycles within a normal range, sad/okay/glad. Major depression has bigger distances, between normal and really sad. Bipolar has the biggest distances. Bipolar I ranges from really sad to really really up, with more time spend down than up. Bipolar II moves the base line down from bipolar I. It goes up, though not so far, and way, way down, lower than the others.
There are other aspects to mood disorders, affecting thought, desire, motivation, energy, sleep, digestion, appetite and even physical pain. But this astonishingly short answer says way more than your common perception that depression means you are sad; bipolar means you are crazy.
Since I regularly write about these and the other mood disorders in Prozac Monologues, it may be helpful to give the longer answer here. So today begins another three-part series. I do seem to like these three-part series. Things stretch out when I want to make Prozac Monologues both clear and entertaining -- though I suspect that it's mostly people with diagnoses who get the entertaining part.