Fuse is what Insomniac CEO Ted Price calls "co-opetitive," a variant on the Gears of War 'Horde' mode. Players must survive twelve waves of increasingly more difficult enemies – the exact configuration of each wave is randomized, though you're always guaranteed two boss battle waves.
Price calls Echelon "co-opetitive" because while you're ultimately working together to ensure group survival, the constant money drops – essentially experience points – are something teammates will scramble to grab. With collected cash, players can purchase and upgrade abilities, and progress in Fuse carries across all game modes.
"Part of our goal here is to make this a friendly game – if you've had experiences in multiplayer games before where you jumped in and had your head blown off in the first couple seconds, we don't want that to happen in Fuse," Insomniac CEO Ted Price told Joystiq. "We want Fuse to be a game where you're actually working with players and competing for cash and status. But it's always a friendly competition."