Mmmmmm, a dead guy that was abused as a child? OR maybe hes an infused entity of all the evil business men in the world! :D And now hes just a walking hollow shell, forever on the lookout for someone he might sell a timeshare to.
Another guess might be that hes actually a hobo in disquise (or however you spell that XD) hoping to scare people into giving him 20$. ewe

wow EASY! he is a person/ghost(idk some spectral thing) who dwells somewhere in europe he is 7-8ft tall tendrils coming out of his back he also leaves notes to his victims&if u find all 7-8 (i'm thinkin maby its six but) well u are spared (mabey) &if u dont u die (he ki,lls you)

He is a mythical entity created on the internet like Jeff the Culler and The Rake,and is a very popular internet mythos.He was created by Marble Hornets,with his storyline dating back to Germany as Der Grosseman.He is seen often times in old pictures,especally family ones.He is a tall man,appearing to have the ability to change his height to further scare his victims.He wears a black suit,white button-up shirt,and a red tie.It is debatable wether or not this is an actual suit,or a part of Slender's skin.He has no facial features,and no indincation they ever existed there,some also say he has no ears.He acts differently according to the victim.He is friendly and welcoming to children,and acts passive agressivly more to adults,usualy stalking victims of either age for years at a time,causing them to go insane,and essentially tear their own lives apart.After this,the victim is never seen again.

They say he can teleport.His normal domain is a place with tall trees and secluded areas.They say,however,that he has a fourth dimention he can travel to.As he was created on the internet,it is completly unsafe to say doesn't exist.It is proven that,if the human mind believes in or fears something enough,the the fear or object takes on a real form.So,in small ways,The Slender Man can and definately does exist outside of our paranoid minds looking outside the window.