‘Normal’ is a term widely used to refer to well-balanced skin. The scientific term for healthy skin is eudermic.
‘Dry’ is used to describe a skin type that produces less sebum than normal skin. As a result of the lack of sebum, dry skin lacks the lipids that it needs to retain moisture and build a protective shield against external influences.
‘Oily’ is used to describe a skin type with heightened sebum production. This over production is known as seborrhea.
Combination skin is, as the name suggests, skin that consists of a mix of skin types.