Bosses are aggresive, difficult mobs that pose a challenge to players. Bosses tend to have much higher health, deal more damage, and attack the player constantly. There are only 2 bosses in Minecraft, (excluding the story mode content). Each having their ways of spawning and attack methods. Elder Guardians are said to be "boss" in the Pocket Edition, as they tend to be quite a challenge to kill in survival.
Killing a boss, will either give the player a rare drop, or change the minecraft world. All bosses will always drop expierience points.