According to the DSM-TR-#296.0-296.89 (Diagnostic and Statistical Manual of Mental Disorders), the clinical features of bipolar disorder are manic episodes, depressive episodes, and mixed-manic episodes. Bipolar disorder is considered a "cyclic" disorder in which the person cycles through manic, normal, and depressive states. In order to be diagnosed, from a clinical perspective, a patient must have suffered from "at least one manic or mixed-manic episode during the patient's lifetime."
Manic refers to a highly elevated mood, energy level, and agitation. Depressive is the exact opposite of manic. In a depressive state, a patient will feel extreme sadness, loss of interest, and loss of energy.
The DSM notes that genetics play a large role in bipolar disorder . A patient's family history helps with a clinical diagnosis of bipolar disorder.
Below is a link to the full analysis of bipolar disorder in the DSM.