Rather than a single disorder, Bipolar Disorder is more of a group of mood disorders. Bipolar Disorder used to be called Manic-Depressive Disorder because of its most obvious characteristics, mood elevations (mania) and depressions.
This disorder usually onsets in the late teen years or early adulthood. It is incredibly rare to find a child that exhibits symptoms and meets the strict criteria to be diagnosed with Bipolar Disorder.
People with Bipolar Disorder will commonly experience a normal, calm mood between episodes of extreme moods. In some people, mania and depression can alternate very quickly - known as "rapid cycling." In order for a case to be called rapid cycling, a person must have had at least four episodes within an entire year.
An important note is that antidepressants will actually have adverse effects on those with rapid cycling (unless the treatment includes mood stabilizers as well).
Although depressive episodes may include feelings of sadness, loneliness, anxiety, and irritability, one of the major issues is suicidal thoughts - individuals with Bipolar Disorder are at higher risk of attempting suicide. Manic episodes may include an irritable mood, extreme/unusual behavior, and even a feeling of being "chosen"; people in manic states may suffer from impaired judgement and shortened attention spans as well.
It is certain that Bipolar Disorder can be genetically influenced, but it's agreed that it does not simply originate from a single gene. Also, the National Institute of Mental Health has stated, "There is no single cause for bipolar disorder — rather, many factors act together to produce the illness." A popular theory known as the "kindling" theory suggests that it is both genetically and psychologically based - it develops over time through stressful events and such.
Two main techniques are used to treat Bipolar Disorder - pharmacological (medicine) and psychotherapeutic (personal intervention/therapy) techniques.
Many cases of Bipolar Disorder employ the use of mood stabilizing medication for the treatment. These are not the same thing as antidepressants; in fact, the use of antidepressants to treat Bipolar Disorder is debated by researchers. On the flip side, a combination may be used to alieve major depressive symptoms.