Garlic powder, and its cousin, granulated garlic, are both dried, ground garlic cloves. The difference in these forms is merely texture, garlic powder having a flour-like consistency and granulated garlic being coarser, like fine cornmeal. Most reputable manufacturers sell 100% pure versions of both, but occasionally you will find additives to improve flow or prevent caking.
Both ingredients are frequently used in BBQ and grilling rubs.
You can substitute garlic powder for granulated garlic and vice versa, but watch the amounts: because the powder is more concentrated, it packs a stronger punch. Fresh garlic is not a good substitute for either.