Quoting from the game's description: "Unleash your primal cube instinct! Run from the biggies and munch on the small'uns."
In the game: there is a forest, a city with eight skyscrapers, and a beach with an ocean. Even though the player's cube's color is picked randomly, they can change the color and/or the texture of their cube (but they need to be unlocked first).
In Cube Eat Cube, the player uses the mouse to move and the space-bar to split. The bigger the player gets; the slower they move. But you can split into cubes too.