The three main things that make bipolar disorder different from ordinary mood swings are:
Intensity: Mood swings that come with bipolar disorder are usually more severe than ordinary mood swings.
Length: A bad mood is usually gone in a few days but mania or depression can last weeks or months. With rapid cycling, moods last a short time but change quickly from one extreme to another. With rapid cycling, “level” (euthymic) moods do not last long.
Interference with life: The extremes in mood that come with bipolar disorder can severely disrupt your life. For example, depression can make a person unable to get out of bed or go to work or mania can cause a person to go for days without sleep.
For more detailed information on the types of bipolar disorder and their symptoms, read the Mood Disorders section of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), which may be available at your local library.