Age of Empires 2: The Conquerers includes new maps, units, and campaigns.
In the game there are new units, new factions, and new campaigns. There are I assume at least 30 or 40 new units. There are I think at least 4 new factions, 4 new campaigns, and another campaign that includes a bunch of famous battles. There are new maps to the custom games. The game includes Mongolian campaigns, Aztecs, and much more. There also scenario battles like Hastings, Mankizert, Noryang, and Againcourt.
AOE 2 The Conquerers expansion offers a lot. It's great, period. I recommend that Age of Empire fans get this game expanding more gameplay (See AOE 2 for full review since this is an expansion.)