Chronic depression is the full blown effect of depression that can have a greater amount of symptoms that needs to be treated by a professional, or perhaps also with medication.
Dysthymia is a far lesser form of depression; in that its not as full blown as the 1st, and has lesser symptoms. Although they are lesser, they shouldn’t be ignored. If you think you have either, speak with your doctor asap. The dangers of Dysthymia is that you can develop Depression on top of it! They call that double depression. You wouldn’t want that.