What causes psoriatic arthritis?

My friend is 47 years old and it looks like he is suffering from psoriatic arthritis. I would like to know what kind of illness is that, what causes it, some info about it. Can you help me out? I guess he is a bit embarrassed about it and doesn't want to talk much about it.

Psoriatic arthritis is a condition most simply explained by the term skin inflammation (with joint inflammation too). Main and most visible symptom are red, patchy skin areas, sometimes with scaling. Most common parts of the skin affected by this condition are knees, elbows, navel, and scalp. Even genital and anal area might be attacked. People that suffer from psoriasis usually have joint arthritis which results in psoriatic arthritis.