Psoriasis is a persistent and chronic skin disease which has a tendency to be genetically inherited. The word comes from ancient Greece, and means "to itch". Red eruptions appear on the surface of the skin and begin to itch. These areas form plaques over the reddened lesions. The plaques resemble multi-layered scales of skin. Psoriasis varies in intensity from a few random spots to a massive outbreak covering the entire body and requiring hospitalization.

In some cases, psoriasis is so mild that people don't know they have it. At the opposite extreme, severe psoriasis may cover large areas of the body. Psoriasis cannot be passed from one person to another, though it is more likely to occur in people whose family members have it.

Plaque Psoriasis: The most common type of psoriasis is called plaque psoriasis. It is characterized by raised, inflamed (red) lesions covered with a silvery white scale. The scale is actually a buildup of dead skin cells. The technical name for plaque psoriasis is psoriasis vulgaris (vulgaris means common). Plaque psoriasis may appear on any skin surface, though the knees, elbows, scalp, and trunk are the most common locations. 

Guttate Psoriasis: Guttate psoriasis is characterized by small red dots (or drops) of psoriasis. It often appears on the trunk, arms and legs. The lesions may have some scale. Guttate psoriasis frequently appears suddenly following a streptococcal infection or viral upper respiratory infections. There are also other events that can precipitate an attack of guttate psoriasis: tonsillitis, a cold, chicken pox, immunizations, physical trauma, psychological stress, illness, and the administration of antimalarial drugs. 

Inverse Psoriasis: Inverse psoriasis is localized in the flexural surfaces of the skin, e.g., armpit, groin, under the breast, and other skin folds. Typically, it appears as smooth inflamed lesions without scaling and is particularly subject to irritation due to rubbing and sweating.

Erythrodermic: Ordinarily erythrodermic psoriasis appears on the skin as a widespread reddening and exfoliation of fine scales, often accompanied by severe itching and pain. Swelling may also develop. 

Pustular Psoriasis: Pustular psoriasis is characterized by pustules (blister-like lesions of noninfectious pus) on the skin. The pus consists of white blood cells in the skin. It is not an infection, and it is not contagious. It may be localized to certain areas of the body, e.g., hands and feet, or it can be widespread. It tends to go in cycles of: 1) erythema (reddening of the skin), 2) formation of pustules, and 3) scaling of the skin. 

Psoriatic Arthritis: About 10 percent of the people who have psoriasis on their skin also develop a form of arthritis called psoriatic arthritis. Psoriatic arthritis causes inflammation and swelling primarily in the hands, feet or in larger joints such as the knees, hips, elbows, and the spine. It may cause stiffness, pain, and joint damage. 

Scalp Psoriasis: Scalp psoriasis affects at least half of all people who have psoriasis. Generally, the scalp will have plaque psoriasis characterized by elevated, inflamed, lesions covered with silvery white scales 

Nail Psoirasis: Psoriasis can affect both the toenails and fingernails. Commonly it appears as pits in the nails of various size, shape, and depth. Sometimes the nails develop a yellowish color and become thick. The nails may crumble easily and be surrounded by inflammation. Another possible symptom is detachment of the nail from the nail bed.