Wikia

Bosses are similar to enemies, except that they are usually fought at certain points of games. Bosses are usually fought halfway through a game, certain spots in a game, or on the last level of a game. Bosses have to be defeated to advance farther in the game. Bosses are usually bigger then most enemies, but can be smaller. Most boss fights are fought in a single a room and a health bar is sometimes shown on the bottom of a screen, although in most Nitrome games bosses are not shown with a health bar. Bosses are stronger then all other enemies in games, unless there are other, more powerful bosses later on in the game. Not all Nitrome games have bosses. However, bosses are always found in main games: mini games do not have bosses.