Psoriasis is a chronic inflammatory skin disease that follows a relapsing and remitting course. Plaque psoriasis (psoriasis vulgaris or chronic plaque psoriasis) is the most common clinical form of the condition, accounting for about 90 per cent of cases. It is characterised by well-defined, thickened, red plaques covered with silvery scales that are readily shed. A plaque is a raised patch on the skin more than 2cm across. On black skin plaques appear dark red and the scale appears greyish.

Psoriasis plaques can occur almost anywhere on the body but the most commonly affected areas are the scalp, the extensor (outside) surfaces of the limbs (typically shins and elbows) and the lower back. The plaques tend to be symmetrical and can crack and bleed. Many people find their plaques itchy. The extent of the disease is variable.