Bipolar disorder is a mental illness involving episodes of serious mania and depression. The person's mood usually swings from overly "high" and irritable to sad and hopeless and then back again, with periods of normal mood in between.
Bipolar disorder typically begins in adolescence or early adulthood and continues throughout life. It is often not recognized as an illness, and people who have it may suffer needlessly for years or even decades.
Effective treatments are available that greatly alleviate the suffering caused by bipolar disorder and can usually prevent its devastating complications. These include marital break-ups, job loss, alcohol and drug abuse, and suicide.
Above text courtesy of the National Institute of Mental Health