Herobrine(Pronunciation) is the subject of a community-made "creepy-pasta" (creepy story). He is one of the major community icons of Minecraft. Herobrine has not been present in any version of Minecraft.
"Canon" of Herobrine is widely regarded as the first image/story ever posted about Herobrine, as well as the Brocraft stream. Anything posted after these two events are considered as fanmade adaptions/variations.
He is Very Creepy And Scary Has White eyes Herobrine is Notch's dead brother, somehow embedded into Minecraft. While this is the "canon", this is also completely false in reality, as Notch has no brother.
Ghost aside, there are two plausible canon theories remaining.
Herobrine shows a lot of characteristics of being a form of virus, such as manipulating game worlds, deleting threads and sending messages through the Minecraft Forums. Perhaps also hacking into Notch's email and Twitter to reply on his own existence. Or, Herobrine is simply a figment of the users imagination. All users the user talked to, including Notch's email reply, could simply be the creation of the player's mind. This would also explain Notch's rejection of the canon, as showing the story is all in the user's head.
Herobrine is near-certainly inspired by Slenderman, who is also the inspiration behind Endermen.

herobrine is also from a game called Creepy Pasta