What is (or rather, was) Herobrine?

I'm pretty sure this line's main purpose in the version 1.6.6 changelog is to mess with us, but still, it's there:

• Removed Herobrine

Curse's Minecraft Wiki does no longer want to host information about it, so — what is this Herobrine thing that would have been removed?

(Also see this question asking whether or not it's ever been added in the first place.)

Herobrine was a fictional character that was said to 'haunt' Minecraft. People would fake screenshots saying he followed them into caves or whatever. He was said to be Notch's dead brother (who doesn't exist).

With the mod that allows you to bring fake NPCs into the game, it became more common to put fake Herobrines into the game.

He does not exist and is a joke. This was Notch's response - trolling.

There is a ton of backstory and stuff here.

Quick repost of the original source of Herobrine, from the link @Communist Duck provided (in case that link ever goes down):