This creature is often described as being fairly tall and inhumanly thin. He is also said to have many arms(or tentacles in many cases). In almost every sighting, he's wearing a suit. I'm not sure why he's described as wearing a suit, but I'd like to think he got the idea from Barney Stinson.
Slendy has been spotted mainly chasing lost hikers or small children. I don't know if there is any correlation between the two, but that's his main demographic nonetheless. The game "Slender" has caused a big spike in his popularity, and centers around your character being chased by him while you find a bunch of pages in the woods.
This post wasn't the best I've done, but probably because instead of doing research and checking facts, I was busy hiding under the covers after hours of playing Slender Man related games. After a while, I expected to find random pieces of paper taped to my walls and see well dressed monsters in my house.
"Honey, who's at the door?"