Enderman. I'll describe him, first.

Imaging the regular minecraft player. Now, make his legs and arms longer and thinner. Make him completely black, face and all. And give him white eyes. Yeah. Minecraft just got real.
See Enderman here

They're coming in 1.8. I'll explain them the best I can, check Notch's blog or youtube it for better info.
Enderman is regularly a nice little mob, but he wants to be just like daddy, so he carries blocks around and places them elsewhere. So don't freak when your chest containing your life savings is missing. When you don't see Enderman at all it's cool. But then comes the part where fear comes in. You see an Enderman, I mean stare right at him, you are screwed. So back away from him. Now. Or perhaps you want to die. If you turn around, you'll hear footsteps. Now turn back. Enderman is there. Oddly, he's closer. Repeat, and soon he's in your face. Now, turn and don't look back. Boom. Within seconds, your dead.

Here's the easier, more understandable way. It comes from a Chuck Norris joke.
"If you can see Enderman, he can see you. If you can't see Enderman, you may be seconds from death."

Or i'll just explain how Enderman works. He picks up blocks and places them at random. When your character stares at an Enderman, the Enderman freezes. If you stop looking at him, he will walk (very quickly) towards you. If he is far away, he will TELEPORT to you. When he gets close enough, he attacks, with heavy damage.

That's really it as far as is known. Are you not scared? Go look at footage of Endermen on youtube here (fanmade, as far as I know, but it looks real enough).
Or imagine Endermen in canverns, at night, or just dark places.

That's scary.

UPDATE: Just found out Enderman doesn't attack you if you wear a pumpkin on your head. (lol)