Campaign features coop but it also has an extensive level editor allowing you to make your own coop levels or download pre-made ones etc. The game allows 4 players on arcade mode and 2 in campaign mode.
The player controls a flying aircraft made out of polygons, like everything else in the game. As the player destroys enemies, their debris fall off the screen below. If the player manages to touch these pieces of debris with their ship before they disappear from the screen, these pieces affix themselves to the player's plane in the positions they were caught, providing extra mass and firepower.
The extra parts, which reanimate and continue to fire their own unique weapons, can serve as shields against enemy projectiles (each part breaks into pieces and falls off upon hit), and all of the augmentations can be retracted into the original ship.
There are a wide variety of different enemies, which provide varying patterns of fire and numbers of points. [WIKI]