Psoriasis is an autoimmune disorder in which your skin produces skin cells much faster than normal. This causes them to build up into thick, flaky, itchy, inflamed patches. There are several types of psoriasis, including plaque psoriasis, guttate psoriasis, nail psoriasis, inverse psoriasis, and several other types that are rare. Common areas of the body where psoriasis develops include the scalp, torso, arms, thighs, armpits, groin, under the breasts, between the buttocks, and on the fingernails and toenails. There’s also the very painful psoriatic arthritis, in which both the skin and the nearby joints are stiff and inflamed.