Halo is a massive ring world created by the anceint race called The Forerunners". It was made as a weapon against the Flood; A parasitic organism that seeks to consume all life in the galaxy. When triggered, the Halo rings would fire, wiping all sentient life from the universe. The main protagonist of the game series, Master Cheif, seeked to destroy the halo rings before The Covenant could use them against Humanity. The Covenant are a collection of many races of aliens. In the first Halo game, you would fight Grunts, Elites, Hunters, and Jakals. These races were led by The Prophets; a race of alien beings that created this religion of the Covenant. These aliens belive in The Great Jorney, this jorney would happen when they fired the halos. The Marines of the U.N.S.C.(United Nations Space Comand) Stopped the aliens from fireing Halo. This is just a very brief story of the first Halo game. For more info, see the Halo Wiki.