The Slender Man was created at the Something Awful Forums in a thread entitled "Create Paranormal Images." He is described as wearing a black suit strikingly similar to the visage of the notorious Men In Black, and as the name suggests, appears very thin and able to stretch his limbs and torso to inhuman lengths in order to induce fear and ensnare his prey. Once his arms are outstretched, his victims are put into something of a hypnotized state, where they are utterly helpless to stop themselves from walking into them. He is also able to create tendrils from his fingers and back that he uses to walk on in a similar fashion to Doc Ock, the Spider-Man villain in the Marvel Universe. The superhuman stretching ability could also be seen as a similiarty between himself and Mr. Fantastic of the Fantastic Four. Whether he absorbs, kills, or mearly takes his victims to an undiclosed location or dimension is also unknown as there are never any bodies or evidence left behind in his wake to deduce a definite conclusion.
Not much is known, as no specific information has been found about origins, but his objective and habitat are made very clear. He has the need to kidnap children, and is seen right before the disappearance of a child or multiple children. He seems to prefer fog enshrouded streets and wooded areas as a way to conceal himself from being noticed. It should also be noted that children have been able to see him when no other adults in the vicinity could. Children also have dreams or nightmares concerning The Slender Man before their disappearance. Confiding these stories to their parents are met with the usual parental admonition: overactive imaginations.
It is also said that the Slender Man just doesn't attack Children, there have been reports of Adults being attacked. Some say that once they manage to get away from the Slender Man (If possible) they grow into paranoia, eventually committing suicide in fear of seeing the monster again. Pictures that are taken of him make him seem like he shows no sign of aggression, so it might be possible to be able to walk up to the creature without being attacked right there and then.