Young ginger, sometimes called spring ginger, has a pale, thin skin that requires no peeling. It's very tender and has a milder flavor than the mature roots. Young ginger can be found in most Asian markets during the springtime.
Mature ginger has a tough skin that must be carefully peeled away. Once it is removed, all the delicate, desirable flesh inside can be used in cooking.
Stem ginger has pink-tinged tips and is available in Asian markets in the spring and fall. Stem ginger is mild in flavor and usually doesn't need to be peeled. It should be used immediately.
In addition to fresh ginger, and the dried, ground spice, ginger comes in several other forms, including ginger juice, which is simply the juice extracted from the root. It's available in some specialty markets and natural food stores. Crystallized or candied ginger has been cooked in a sugar syrup and coated with coarse sugar, whereas preserved ginger has been pickled in a sugar-salt mixture. These types of ginger can be found in Asian markets and many supermarkets. They are generally used as a confection or added to desserts. Pickled ginger, most often used as relish or garnish for Asian dishes, is available in Asian markets.