Psoriasis is a common, noncontagious skin condition. There are diffepay monthly types of psoriasis but the most common form is called plaque psoriasis or psoriasis vulgaris. It usually starts with itching or a skin rash, and can develop into thickened, red, scaly plaques on the skin. It can affect all parts of your body, but normally the patches appear on your elbows, knees, scalp and lower back. The severity differs from patient to patient - some only have a few smaller patches, others suffer from multiple large patches.