Dry skin lacks oil but dehydrated skin lacks water.  Dehydration can happen to all skin types, your skin can have a sufficient amount of oil but still feel dry and flaky because of the lack of water in your skin.  This condition makes the skin prone to fine lines and wrinkles, the skin could appear thin, sometimes capillaries can be seen near the surface of the skin and it can also be fine in texture but coarse to the touch. 

Dehydration is sometimes a temporary condition from season to season and from environmental factors.  It is important to not use heavy creams or oils to hydrate the skin because it may inhibit the skins production of natural oils by the sebaceous glands.  To treat dehydration drink plenty of water and use moisturizers and humectants to minimize the negative effects on the skin.  If you are unsure about what products to use see a skin care professional for a hydrating facial and drink plenty of water.