Disks act as cushions between the vertebrae in your spine. They're composed of an outer layer of tough cartilage that surrounds softer cartilage in the center. It may help to think of them as miniature jelly doughnuts, exactly the right size to fit between your vertebrae.
A bulging disk extends outside the space it should normally occupy. The bulge typically affects a large portion of the disk, so it may look a little like a hamburger that's too big for its bun. The part of the disk that's bulging is typically the tough outer layer of cartilage. Usually bulging is considered part of the normal aging process of the disk and is common to see on MRIs of people in almost every age group.
A herniated disk, on the other hand, results when a crack in the tough outer layer of cartilage allows some of the softer inner cartilage to protrude out of the disk. The protrusion of inner cartilage in a herniated disk usually happens in one distinct area of the disk and not along a large component of the disk, which is more typical of a bulging disk. Herniated disks are also called ruptured disks or slipped disks.
Bulging disks are more common. Herniated disks are more likely to cause pain. But many people have bulging disks or herniated disks that cause no pain whatsoever.