Gladly.
First you need to know of the first known image of the Slender Man.
If you look in the left corner, by the tree, you'll notice a oddly portioned man. This is one of two recovered photographs from the Stirling City Library blaze. It's notable for being taken the day which fourteen children vanished and for what is referred to as 'The Slender Man'. Deformities on 'The Slender Man', from this point on just called Slender Man, cited as film defects by officials. Fire at the library occurred one week later. The actual photograph confiscated as evidence.
From this moment onwards, and the discovery of this image, and people began to notice more and more that the Slender Man had showed up before, in the past.
He is an enigmatic figure, most often seen as a tall, extremely thin man with long, strange arms, and a face that no two people see the same way (if they see any face at all) wearing a suit. Where he comes from is as much a mystery as what he wants. All that is known is that there is evidence of him existing for far longer than one would expect. Those who see him often wind up missing—or worse—with their mutilated bodies impaled upon a tree, and their organs removed and then replaced systematically.
His presence is associated with paranoia and sometimes a strange sickness, and those who see him are frequently found to be maniacally writing strange messages, and drawing mad scribbles of a dark, faceless figure.
It has been advised -not- to go seeking out more information on the Slender Man... as to avoid attracting... unwanted attention.
There are four main projects seeking out information on the Slender Man. Marble Hornets, Everyman HYBRID, Tribe Twelve and Just Another Fool, are most closely associated with the Slender Man mythos, along with a series of strange stories from people who appear to have seen him, as well as photographs by people who have not been heard from since. Rudimentary internet searches can find all of these, or you could try the TVTropes 'Slender Man Mythos Stories' page.
((Or, for those of you who are now -REALLY- freaked out, let me calm your nerves with this:
The Slender Man Mythos is a 'Creepy Pasta', or scare-factor copy-paste that was created by Victor Surge on Something Awful, and quickly mutated into a major Meme and what you see before you today. Don't turn the lights off kids.
You never know what's gonna go bump in the night.))