Dragon Age III will be set in Thedas, the fantasy world in which the two previous games are set. The game will cover more geographic territory than its predecessors, with one map being described as four to five times the size of Ferelden, the setting of the first game in the series.
The plot will involve an all out war between mages and templars, the foundations of which comprise the main plot of Dragon Age II. Also, along with the larger world, the player will be able to travel to places as far as Orlais and Tevinter to calm or encourage the fighting.
BioWare is planning on fusing elements of both earlier games in the series, Dragon Age: Origins and Dragon Age II, into the creation of Dragon Age III.
Combat is also expected to differ somewhat from its predecessors and focus more on a player's ability to prepare, position and form a cohesive team with his or her party members, requiring fewer repetitive finger strikes but better thinking.
Ray Muzyka, BioWare's CEO, said in an interview with Wired.com that Dragon Age III would be influenced by more open world games, such as The Elder Scrolls V: Skyrim, which BioWare is "checking out aggressively."
Source: Wikipedia, "Dragon Age III", available under the CC-BY-SA License.