There was a glitch in the early stages of the game where the player suddenly started losing health until death, the coding for this glitch contained the words "hero" and "brine", leading people to believe it was a mob named "Herobrine". It spawned many CREEPYPASTAS and stories, but Notch has stated that Herobrine does not exist. However, he can be seen in different artwork for the game, but this is likely a joke. In many patch notes, it will say, "removed Herobrine", this is because Notch wanted people to know that Herobrine isn't real.
The game allows you to choose 4 different game modes: creative, in which the player can fly and have unlimited resources and survival, where the player needs to survive and obtain their own resources. Adventure, where blocks are unbreakable, unless you have certain tools. Spectator, where you can fly, but not interact with the surrounding environment.