There are actually several disorders which are classified as "bipolar disorders" in the 4th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). These disorders are mood disorders characterized by shifts between mania or hypomania (an elevated mood) and depressed mood features. The bipolar disorders are: bipolar I disorder, bipolar II disorder, cyclothymia, and bipolar disorder not otherwise specified.
For some individuals, the manic or hypomanic episodes occur at the same time as the depressed mood episodes. These are called "mixed episodes," and individuals who experience these types of episodes can still be diagnosed with a bipolar disorder.