What is the difference between sebhorreic dermatitis and psoriasis?

Sometimes overlap. Seborrheic dermatitis of the scalp can often overlap with scalp psoriasis in a disease termed sebopsoriasis. Both conditions are from scalp inflammation. The treatments are often quite similar using topical steroid solutions/shampoos/foams and keratolytic (scale removing) shampoos and topicals. See a dermatologist to see if you have one or the other or the combined form (sebopsoriasis).