Dear Ms. Holder,
Bipolar Disorder is one type of a category of emotional illness called Mood Disorders. Bipolar Disorder is characterized by the presence or history of both depressive episodes and manic or hypomanic episodes. In adolescents, the so-called manic component of the condition is usually typified by volatility, reactivity, extreme irritability, etc., rather than by the kind of mania that we see in the adult population.
Typically, bipolar disorder is treated in part by a combination of antidepressant and mood stabilizing medications, and possibly (for a time) anti-psychotic medication.
You can search this site for information on bipolar illness.
Mental Health Net has a good information page, too, at the following web address:
http://bipolar.mentalhelp.net/
Best wishes to you and your son.