Many medical disorders have both serious and mild versions. Bipolar Spectrum Disorders include the following: Bipolar I (with both severe depressive and manic episodes); Bipolar II (severe depressions and milder forms of mania: hypomania); Cyclothymia (mild depressions and hypomanias) and childhood onset bipolar disorder. Childhood onset bipolar disorder is listed separately because it often has a somewhat different set of symptoms (e.g. mixed manic episodes: very high energy, agitation, decreased need for sleep, extreme temper tantrums lasting for hours, severe depression, irritability and a significant risk for suicidal attempts; this is the most commonly seen version of mania in children). Also, unfortunately, the medical treatment of childhood onset bipolar is much more challenging (medications that can be very effective in treating the later no-set bipolar disorders are only marginally effective in children suffering from this condition).
Cyclothymia lies on the mild side of this spectrum and Bipolar I and II on the severe end. Another thing that is common to all of these versions of bipolar disorder is a family history of bipolar and other severe mood disorders. These conditions are felt to be genetically transmitted, and thus are seen in many blood relatives.
Some researchers include other mood disorders in this spectrum (although this is not firmly established): Bipolar III (only recurrent, severe depressions, but mania can be provoked if the person used cocaine or methamphetamine or is taking antidepressants). Note: antidepressants can ignite manic episodes in bipolar patients and thus these drugs generally are not used in treating bipolar disorders (there are rare exceptions). Another condition is Hyperthymia: this is a non-episodic set of traits that include: up-beat mood, self-confidence, energetic, sometimes impulsivity and a decreased need to sleep (e.g. routinely sleeping 5 hours a night and not experiencing daytime fatigue). Obviously this is a condition for which almost no one seeks treatment. However, this can be seen in blood relatives who suffer from more severe forms of Bipolar Disorder.