Uni-polar is when people only experience one of the extremes, and then alternate with being stable.
i.e. depression and then stable and back to depression. or Mania, then stable, then mania.

If you are Bipolar, you go through Depression, stable and Mania.

Bipolar I is characterised by extreme mania and some stability (Depression doesn't feature too much) and the cycles tend to be longer.
Bipolar II tends to be shorter cycles, (Some rapid cyclers can change multiple times in a day), it appears to be a lot of depression, with stable times, and a much milder form of Mania, referred to as hypomania.

Everyone experiences this illness differently - there is no textbook answer.

If you have never had a hypomanic or manic episode, it is technically not possible to diagnose you as bipolar. You would then be uni-polar - of which Depression is one type.

It is also worth investigating Borderline Personality Disorder (BPD) which has very similar symptoms to Bipolar. There is a whole forum dedicated to this too.
On the top of many of the forums there are some sticky threads, with great information on the various conditions.
At the top of the whole forum is a link called Conditions & Disorders - there's a wealth of info there and it's definitely worth reading.