Demos are a type of game Nitrome releases less often than main games that usually contain some art and a mechanic to be used in an upcoming game.
Demos exist and are released by Nitrome for feedback on the particular mechanic used in the Demo. Demos will likely contain art to be used in the game. Demos will still have a challenging aspect Main Games have, but will probably be shorter in levels than regular games. Not all upcoming games will get a Demo. If a game does receive a demo, the final game will have differences that range from small to massive.