The user N1KF has suggested moving this page to Boss: Some bosses come in worlds that aren't boss levels. This topic is more about the boss itself than the level type.
Bosses are a type of interactive level that can be considered live puzzles. They feature competitive play between players. Boss are often seen alongside minigames and art. Prizes can range from simple congratulations in the form of a coin and crown to running the boss level with edit rights.
The owner or another editor creates a large rectangle or square in the map to serve as the playing field, and creates a Gate (typically at the top) where the players can wait for the next round to begin. Bosses are usually accompanied by lobbies, where the player can practice various puzzles.
In a Color Boss, the playing field is split into two, with each side being constructed of Doors of a different color. Each side features identical minigames and challenges, but are typically mirror images of each other. At the end of the challenges is a key for the other side's doors. Like Normal Bosses, they are accompanied by lobbies where players can wait for the next round.
In a Copy Smiley boss, the playing field is normally arranged so each player has their own area to sit it. When all of the areas are filled, the owner or the person who is 'bossing' will change a smiley that all active players have to copy. Normally there would be a certain amount of blocks under each area representing how many 'lives' each player has. The last person standing gets the prize.
In a Boss Level, the goal is to be the last man standing. Prizes range from the Crown, to the level code or edit access, but the Crown is the most common. Typically you have to win a boss a certain number of times before the grand prize is given. This is controlled by a system of coins, coin doors, and gravity arrows. Boss levels involve multiple people trying to complete any of the level types listed here (as well as some Common puzzles) as fast as possible or before anybody else. Loss of a boss battle will often result in your falling to the bottom of the playing field, which may contain gravity arrows to push you back into the lobby. Unique to boss battles are active platforming, where if you aren't fast enough bricks below you start to disappear. Before the bricks disappear, the boss usually gives a warning by turning them red or another color.
In a Color Boss, the pool of players is divided between each half of the playing field. The players will then compete to try and reach the key at the end first, which will drop the floor out from underneath the opposing team's side. Wins are sometimes tallied with Bots or by hand.
Many players compete to change to the smiley the owner/'bosser' has to win the prize.