What is Proriasis?
Psoriasis is a skin disorder characterized by sudden or gradual development of reddened, thick, itchy patches of silvery scales (raised layers of dead skin flakes) usually in the elbows, knees, scalp, hands, trunk and nails. The skin may blister (raised and bubble like) or have pustules (small, raised, and pus-filled). The nails may show pitting (pin-sized punched-out areas) and the joints may be painful (arthritis).

Symptoms
Skin patches -- red, dry, and covered with silvery scales
Pustules
Blisters
Nail changes
Itching
Patches and itching may involve the scalp
Joint pain (arthritis)

Cause
In a normal person, the new skin cells gradually move from lower layers to the surface (takes about a month), but in Psoriasis the process speeds up to a few days -- which results in piling up of dead skin cells (scales) in patches.
Evidence of genetic factors -- increased HLA antigen (white blood cell or leukocyte surface protein) and a strong family history in Psoriasis sufferers.
The immune system (body's defenses) of disease-fighting cells (i.e., white blood cells and protein antibodies) may also play a role by misunderstanding internal communications and attacking skin and joints -- causing inflammation (irritation, swelling, pain) and damage.