According the National Psoriasis Foundation, psoriasis is a chronic, autoimmune disease that appears on the skin.  It occurs when the immune system sends out faulty signals that speed up the growth cycle of skin cells.  There are five types of psoriasis: plaque, guttate, inverse, pustular and erythrodermic.  The most common form, plaque psoriasis, appears as raised, red patches or lesions covered with a silvery white buildup of dead skin cells, called scale. 

The scalp, elbows, and knees are the most common sites, but almost any part of the skin may become involved.  Psoriasis usually begins in young adulthood, although it can start in childhood or first appear in old age.  In most cases, psoriasis is mild and is limited to a few areas of the body. 

The National Psoriasis Foundation as more information about psoriasis and information about virtually every treatment.