Psoriatic arthritis is a specific condition in which a person has both psoriasis and arthritis. It is an autoimmune disease, in which the immune system is misdirected to cause inflammation of one's own tissues. A person can have psoriatic arthritis without having skin psoriasis. Moreover, the arthritis can precede the psoriasis by months or years, or present after years of psoriasis. You can see the following picture for reference.