Basically, lingering depression.
Dysthymic people tend to feel detached, unhappy, tired, hopeless. While Dysthymic symptoms tend to be less severe than the symptoms found in other, less permanent types of depression, they can linger with a person for their entire life. Usually, in adults, mild depression that lasts for more than 2 years is the definition.
Most suffering from dysthymic depression couldn't tell you the last time they felt truely happy.
Sadly, this is one of the least treated, or diagnosed forms of depression, as the subject tends to believe that these feelings are normal. However, anti-depressants can be very effective in correcting dysthymia.
Everyone said that he always seemed exceptionally detached and unhappy, but nobody knew he was suffering from dysthymia.