Wiktionary
n. A website intended to be offensive or shocking to most viewers, containing frightening and/or extremely distasteful and crude content of a pornographic, scatological, or grotesquely violent nature. In most cases, it displays a particular picture most prominently; some shock sites also show animations or galleries of images, particularly shock images.
Wikipedia
A shock site is a website that is intended to be offensive, disgusting, or disturbing to its viewers. They contain material of high shock value that is considered distasteful and crude; the content is generally of a pornographic, scatological, graphically violent, insulting, painful, profane, or otherwise provocative nature. Some shock sites display a single picture, animation, video clip or a small gallery, and are often passed around via email or disguised in posts to discussion sites as a prank. Steven Jones distinguishes these sites from those that collect galleries of shocking content, such as Rotten.com, as the gallery sites must be actively searched for content.
Some shock sites have also gained their own subcultures and have become internet memes on their own. Goatse.cx featured a page devoted to fan-submitted artwork and tributes to the site's hello.jpg, and a parody of the image was unwittingly shown by a BBC newscast as an alternative for the then recently unveiled logo for the 2012 Summer Olympics. A 2007 shock video known as 2 Girls 1 Cup also quickly became an Internet phenomenon, with videos of reactions, homages, and parodies widely posted on video sharing sites such as YouTube.