Trollocs are human and animal stock mixed together to create a ferocious beast that kills for the sake of killing. They have half-human bodies, but also fur, snouts, beaks, feathers, horns, fangs, claws, or other random animal things. They can outrun a horse in a sprint, but don't have much endurance. Trollocs are pretty stupid and lazy, they see better at night than day, and can smell real well.
Myrrdraal are the more-human throwbacks that come from this little mixture. They look like a person but they have pale skin and no eyes, but still see better than a human. When they "look" at you an overwhelming sense of fear comes over you and freezes you. They can disapear into and travel through shadows, but nobody, not even their creator, knows how.