Dried ginger powder is aromatic, slightly pungent, and hot. It can be used in sweet and savoury dishes. It is believed to help digestion and calm mental activity.
Dried ginger is fresh ginger which has undergone a drying process. The fresh rhizomes are soaked in water overnight after which the outer peel is carefully removed. They are washed again and then sun-dried for around a week. During this period, they are turned periodically. The result is a pale white dried ginger. This dried ginger is then powdered to produce dried ginger powder.
Dried ginger powder is used in northern Indian cuisine in place of fresh ginger. It is also commonly used in desserts.
Ginger powder is available from supermarkets or Indian grocers, but you are advised to get it from a specialist spice merchant. Commercial preparations have often been bleached to produce the pale colour, and lose a lot of flavour because of this process. Look for a fine, fluffy powder with an off-white colour. If the dried ginger powder is really good you can smell the strong aroma even whilst it is still in the pack.
Note that fresh and dried ginger are not interchangeable.