As cheddarbot said, its a site that has goth/alt/emo girls that focuses on tattooed, pierced and heavily modified girls doing nude modeling. It used to be much more selective and had voting rounds for members to amp up their girl of choice during what they called hopefuls to be a next suicidegirl. It somewhat encompasses the whole horror, dark, and BDSM scenes that many are into.
There was some controversy some years ago because girls were entering into modeling contracts with them and it had all these non-compete clauses and stuff that controlled what they could and couldnt do, and how much the suicidegirl name controlled. There were spin-offs, like godsgirls and probably some others that werent as successful, but werent controlling like that. Since girls were getting exposure through suicidegirls, they were being approached by modeling agencies and missing out on what they could do or face legal issues from their contracts. They even put out some really cheesy suicidegirls movies that were on showtime or hbo, but that was right around its peak.
I think it all sort of blew up at one point and whoever was in charge of it all left or had new management put in control. It used to be a pretty exclusive thing to be a part of, at least within the whole alt modeling scene, but now its just a site that focuses on churning out new girls and sets, has divisions all over the world for each country or region, and may have streaming on there like it did at one point, but its not what it once was. Its still popular among these kinds of girls, but its just a massive collection anymore that you can pay to join to have access to like everything else.