It is called Ero Guro, but is often shortened to just Guro. It is a Japanese art form that focuses on the bizarre, gore, grotesque, and mutilation mixed with eroticism. In the West, gory hentai is often referred to as Guro, especially if there is a heavily mutilated anime girl covered in blood, missing limbs (or other various body parts), or spilling organs. Often all three is present and the subject is deceased.
Items that are pornographic and bloody are not necessarily ero guro, and ero guro is not necessarily pornographic or bloody. The term is often used incorrectly by western audiences to mean "gore"—depictions of horror, blood, and guts.
"I walked in on my cousin jacking to a mutilated, dead anime girl. There was blood everywhere!"
"That sounds like Guro dude."
"I seriously think I'm scarred now, it was terrifying."