The dried and ground version of fresh ginger, dried ginger has an earthy flavor that's a natural with other baking spices like cinnamon or nutmeg, and is what give ginger snaps and gingerbread their familiar warm flavor. Though less potent than its fresh counterpart, ground is also used in savory cooking often as a component in spice mixes.
Like all dried spices, ground ginger's intensity diminishes over time, so buy in small quantities (preferably from a merchant with high turnover) and use it up within six months. You can also but dried ginger in pieces; when grated with a rasp-style grater, the texture is slightly coarser than ground, and it has a more potent flavor.
If you're not sure if your ground ginger is still fresh, smell it: It should have an assertive, spicy, gingery aroma.