Dry skin is a minor condition, which shows as hard or flaky skin, and is simply due to lack of moisture in the upper skin layers.
Psoriasis is a rather more serious condition and is caused by the skin “overgrowing” – giving itself layers it does not need, which causes irritation – large red, sometimes scaly areas are the result.
Eczema / Dermatitis
Eczema and dermatitis mean much the same thing, except that dermatitis is caused by something external, while eczema is an allergic condition that does not need an outside substance to trigger it. The rash of eczema/dermatitis typically has dry flaky skin that may be inflamed and may include small red spots. The skin may be cracked and weepy and is sometimes thickened.