Slenderman is a supernatural entity that resembles a very tall and thin man in a black business suit. He has pure white skin with no facial features or hair. He may or may not have tentacles.
Slenderman is a near-complete mystery. His exact nature and motivation is unknown. Speculation ranges from a creepy but well-meaning being who protects children to a stalking horror who torments to death any who capture his fancy. He is attributed with the ability to disguise himself as a tree, conjure heavy fog, and desintergrate anyone he touches. He also eviscerates people, shrinkwraps their organs, and hangs their hollowed corpses from trees. According to the stories, he only exists to people who beleive he does. Somehow.
While he was created whole-cloth as a creepypasta monster, he is very similar to the Germanic fairy tale "Die Grobmann," or "The Tall Man," about a malevolent fairy who lives in the Black Forest.