In Halo: Combat Evolved, the objective was to capture 15 flag points.
In Halo: Reach, this gametype was usually played in an open area, such as Hemorrhage. The objective is to reach the hill marker before other players. The catch is that you have to accomplish this while driving a Mongoose. As soon as a driver tags the goal, it will move to another location, usually far away from the one before. This gives some far-behind players a chance to get back in the game. Sometimes landmines are placed on the track as obstacles.
There is a slight flaw, though. You can get off of your Mongoose and shoot down other players. Although you don't get points for gunning down opponents, it can be an annoyance and disrupt the game to the point where only a few people are trying to win. There is a ten-second limit for traversing on foot, so that get off their Mongoose will have their Mongoose spawn back with them, but they can get off once more.
The team variant of rally.