The National Institute of Mental Health in the USA provides the following excellent short summary of the condition as reproduced below:
Bipolar disorder, also known as manic-depressive illness, is a brain disorder that causes unusual shifts in mood, energy, activity levels, and the ability to carry out day-to-day tasks. Symptoms of bipolar disorder are severe. They are different from the normal ups and downs that everyone goes through from time to time. Bipolar disorder symptoms can result in damaged relationships, poor job or school performance, and even suicide. But bipolar disorder can be treated, and people with this illness can lead full and productive lives.
Bipolar disorder often develops in a person’s late teens or early adult years. At least half of all cases start before age 25. Some people have their first symptoms during childhood, while others may develop symptoms late in life.
Bipolar disorder is not easy to spot when it starts. The symptoms may seem like separate problems, not recognized as parts of a larger problem. Some people suffer for years before they are properly diagnosed and treated. Like diabetes or heart disease, bipolar disorder is a long-term illness that must be carefully managed throughout a person’s life.
(Source: National Institute of Mental Health: http://www.nimh.nih.gov/health/publications/bipolar-disorder/what-is-bipolar-disorder.shtml).
This short video below produced by Youthspace is an excellent resource and should answer most of your questions about bipolar: