Dysthymia is a chronic type of depression that occurs on most days and lasts for a period of 2 or more years. In children and adolescents, mood can be irritable and duration must be at least 1 year. Also, the person has to display at least two of the following symptoms during the 2 year period: poor appetite or overeating, insomnia or hypersomnia, low energy or fatigue, low self-esteem, poor concentration or difficulty making decisions, or feelings of hopelessness.
There are additional criteria, but these give you a good idea of the disorder. For additional information see: http://www.mentalhealth.com