Dry skin is a skin type which is characterized by lack of lipids (oil). It can proceed to dehydration because, as a result of reduced lipids, skin is unable to effectively bind in water and its evaporation from skin increases. Dehydrated skin describes a condition caused by insufficient water content.
The common denominator for dry, dehydrated and sensitive skin is a disturbed moisture balance. When skin lacks the moisture it needs to work as an effective barrier layer, further moisture is lost and it is unable to protect itself against external irritants such as bacteria or pollution. This leads to feelings of tightness and discomfort.