The demo will cost you 1.4GB of precious (or not so precious) hard drive space, and allows players to experience the first two campaign mission, as well as getting to know "Chasms", one of the fourteen multiplayer maps that will be available when the game ships. Dashboard text below:
Halo Wars lets you effortlessly command powerful UNSC and Covenant armies in epic battles. Designed from the ground up for the Xbox 360, experience events in the Halo universe 20 year prior to the events of Halo: Combat Evolved.
Definitely worth taking a look; its real nice to see more and more devs releasing demos prior to the games release date.