Halo 3 is the conclusion to the halo trilogy made by Bungie Studios. (Halo, Halo 2, Halo 3) It is also an FPS (First Person Shooter) like the other two. The campaign mode will finish the story that ended with a cliffhanger from halo 2 (Though I think you need to read Halo: Uprising to find out exact details on the plot between Halo 2 and 3). Besides the new Campaign. Multiple new features are available in Halo 3 multiplayer, such as Forge (sort of a map editor), new Spartan and Elite variants, equipment, new weapons, gametypes, and much more.
Halo Wars is a Real Time Strategy (RTS) game. It is made by Ensemble, a company that has made other RTS games. Halo Wars takes place in the AHlo Universe before Halo 1. It tells the story of when the Humans first met the Covenant. There are many Spartans in Halo Wars, unlike the Halo trilogy, because they were alive prior to the point at the beginning of Halo 1. In halo Wars, you can create your own army made up of Spartans, marines, warthogs, Scorpian tanks, etc and battle the covenant. I don't know if you can actually play as the Covenant however.
Oh, and Halo 3 is released this September 25 as either the standard ($59.99), limited ($69.99), or legendary edition ($129.99). Standard is just the game, Limited is the game and 1-2 bonus discs, and legendary comes with a carrying case, cat-sized spartan helmet, the game, and about 4 exra discs.
There hasn't been an announced release date for halo Wars yet.
Hope this helped.
