A ruff is a high, frilly collar that's worn snugly around the neck, or a similar frill of feathers or fur around an animal's neck. The Elizabethans are famous for wearing ruffs.
If you wear a ruff today, you're probably acting in a Shakespeare play or a movie about sixteenth century England. Some religious figures, like bishops and choir members, sometimes also wear ruffs. They're more common in animals, from dogs to the bird that's actually called a Ruff, described as having a "large collar of ornamental feathers." Ruff is believed to be a shortened form of ruffle.