Payload is a game type featured in Team Fortress 2. It was not originally included in the game, but was added with the first free update.
There are currently seven official Payload maps released by Valve: Goldrush, Badwater Basin, Hoodoo, Thunder Mountain, Upward, Frontier, and Barnblitz. There is also another game mode, called Payload Race, which is a two cart variant of the Payload game mode in which each team pushes its respective cart.
Payload maps are referred to by the game with the pl_ prefix (e.g. pl_goldrush, pl_barnblitz).
Payload features two teams. The BLU team is the attacking team, while the RED team is defending. The attacking team's aim is to push the bomb cart to the defending team's base, while the defending team must attempt to prevent this.
Most payload maps are split into multiple stages, and if the attacking team is successful in pushing the cart to the end of the map, they will proceed to the next stage. If the attacking team wins the final stage, the bomb will explode in spectacular fashion, destroying the geometry around the final point.