Language crafted to create a disturbing picture in the mind of the reader or listener.
The motivations behind creating such imagery vary between being pure shock value and the vector for catharsis.

(my lame) examples
Pure shock value:
Anything that just makes you say "Oh, gross" or "Yuck" or "Gads!" and then you think of it no more. I am having trouble thinking of an example from something I read or heard recently. Which is a point against this kind of foul imagery, as it leaves the reader/listener with nothing. It is ultimately empty and without purpose.

Vector for catharsis
Jonathon Swift's A Modest Proposal uses foul imagery (that of harvesting human babies for consumption to feed a starving population) to get a reaction from the reader (or listener).
At first the recipient is disgusted, then they are horrified, then they ponder the possibility. And recognize the points the author was trying to make.
BAM! A connection between the reader (listener) and the words of a dead man is made. The reader has benefited from the introspection that inevitably comes when your brain is faced with horrific questions it must ask itself (would I do that? could I do that? can I visualize myself in such a desparate situation and go along with that even if society did, too??).
The reader, emotionally disturbed as they might be, has also had a chance to reorder the way they view the world. Once they accept the emotions that spring forth from such cognitive debate, they are relieved and more at ease with who *they* are.
That's a minor catharsis, in my book.