Dry skin has a low lipid content and the skin barrier is impaired. It may be due to ageing, genetics, hormones, environment, diet or a combination of different factors. Careful selection of the most suitable moisturiser is very important.  For example, for acne prone skin moisturisers should be oil free.  For older skin, moisturisers with hyaluronic acid would be better. Often dehydrated skin which lacks water is confused with dry skin which lacks oil. If your skin feels tight especially after cleansing it may be dehydrated rather than dry.