The object of the game is to K/D. K/D stands for kill/death ratio. This determines how well you are and how well you did in a game. Another object is to level up and get different weapons to use. One more object is to just have fun.
Some things that I think are different and the same between the games is that Call of Duty is more realistic. Halo is more cartoonish and not as realistic and fun if you want to play for real. In Call of Duty you can also