Psoriatic arthritis is a type of arthritis. It usually affects people who have psoriasis or a biological family history of psoriasis. Arthritis is a common disorder that affects your joints. It causes pain and inflammation in and around your joints. Psoriasis causes inflammation in your skin. Psoriasis rashes are patches of discolored skin covered with scales. These thick, scaly areas are called plaques. It can also make your fingernails and toenails thicken and look flaky or pitted (covered in tiny indentations).