Slenderman is an urban legend created for a competition on the SomethingAwful forums. He is a tall man in a suit with white skin and no face. Some depictions show him as having black tentacles coming out of his back. The urban legend gained popularity after a game called Slender was released.
Slender man is a fictional character. He is usually used to scare people. For instance, someone threw a Halloween party and this guy wasn't invited. He decided he would dress up as slender man and still go to the party. He photobombed all their pictures by just standing in the background. The next day on Facebook everyone was freaking out because they couldn't figure out who it was because everyone invited was there.