This trailer gives you a good idea of what you get when you buy Fire Emblem: Awakening DLC. Depending on the map or map pack you buy, you'll get vintage Fire Emblem characters, tons of money, rare items, or even items that let you change your characters' classes to the "Dread Fighter" or "Bride." Sometimes you also get really weird dialogue.
Many of these maps, including the Golden Gaffe and the free Champions of Yore 1, are already available; others are yet to be released. Some can be pre-purchased as part of packs with existing maps.