The ghoul of folklore, like the Hollywood zombie, is an undead creature that eats human flesh. The major difference is that ghouls don't generally eat the living. They dig up the bodies of the dead for their nocturnal feasts. If the Hollywood zombie is like the country cousin of the aristocratic Hollywood vampire, then the ghoul is like a dumpster diver. Ghouls take the dead bodies the zombies throw away.
If you stumbled on a pack of ghouls holding a buffet at a nearby graveyard, they probably wouldn't turn on you. You'd be too fresh. Ghouls aren't scary because they're all that dangerous to the living- they're really just creepy.
There is a Scottish folktale about a man who marries a ghoul- not a very observant fellow, you might say- and when he finds out why she sneaks out to the graveyard every night, she does try to eat him. But it's not nutritional ghoulishness- she's really just trying to keep him quiet.
The Caribbean zombie, the folklore ghoul and then the Hollywood ghoul, which quickly became the Hollywood zombie- four distinct yet equally repulsive forms of the walking dead!