Seborrheic dermatitis is a chronic skin condition that causes a scaly, itchy rash. It can appear as a mild, white scaling or a yellowish greasy or reddish scaly rash. Most commonly affected areas are along the hairline, in and behind the ears, and on the eyebrows, around the nose, and chest.
Dandruff is a type of seborrheic dermatitis. It is the scaling of the skin on the scalp. It can be dry or oily and sometimes itchy.
Seborrheic dermatitis involves swelling and redness of the underlying skin (inflammation), while dandruff is just a mild, white scaling of the scalp (no inflammation).