As cathylynn says, there are two typed of Bipolar disorder, Bipolar 1 and Bipolar 2 - Bipolar 1 being more severe in terms of symptoms, and almost always requiring medication - It can easily cross into schizophrenia and medication can help to slow/stop that progression. Bipolar 2 is less severe. It also almost always requires medication, BUT only cyclically. A person with Bipolar 2 may be able to live quite successfully without medication for years at a time.
Another very similar disorder is called cyclothymia. I can only describe it as a kind of "baby bipolar." Similar emotional ups and downs but not nearly as severe or noticeable as actual Bipolar Disorder. It can often be managed through talk therapy, and rarely requires medication.