Slenderman is perhaps the scariest thing the Internet has ever produced.
He is a creature of sheer fear. An abnormally tall, skinny humanoid in a suit, with no face to speak of. What's more, he theoretically has the power to either stretch his limbs, utilize shadowy tendrils which can emerge from him, have multiple limbs or all three, it's not clear. Anyone who would really know is either insane or gone or both. It's not really clear.
Other than his appearance, not much is really known to be certain. We think he likes to hide among trees and has a preference to prey on children. What he does with his "victims" it is pretty unclear.
We have no idea where he comes from, or why he does what he does, or how he does it. He is a living avatar of mankind's fear of the unknown.