A shock image originally from the website goatse.cx, which was intended to be pronounced as "goatsex", although the suffix .cx is no longer in common use when referring to the picture (resulting in the word being pronounced "goat-see". Basically the picture is of a man stretching his anus to frightening length. Like many other shock paraphernalia (such as the infamous 2 Girls 1 Cup)", people will oftentimes prank people by putting a link online of something (such as a sweepstakes or another website) and when someone clicks this link, they will see the image instead of what they were told they would see.
The website had other disgusting images on it until the site was taken down. Since then a website claiming to be Goatse has reemerged.
Unlike 2 girls 1 cup, you will not find very many reaction videos on sites such as YouTube.
If gross images somehow was made into a pro sport, Goatse would be a star player. It WILL scar you for LIFE.