Disks act as cushions between the bones that form the spine. These bones are called vertebrae. Disks have an outer layer of tough cartilage that surrounds softer cartilage in the center. It may help to think of disks as miniature jelly doughnuts, exactly the right size to fit between your vertebrae.

Disks show signs of wear and tear with age. Over time, disks become less flexible. These changes can cause the outer layer of the disk to bulge out. A bulging disk looks a little like a hamburger that's too big for its bun.

A bulging disk doesn't always affect the entire disk. Usually, at least a quarter to a half of the disk's circumference is affected. And only the outer layer of tough cartilage is involved.

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 stick out of the disk. Herniated disks also are called ruptured disks or slipped disks. The whole disk does not rupture or slip. Only the small area of the crack is affected.

Compared with a bulging disk, a herniated disk is more likely to cause pain. The inner cartilage of the disk sticks out farther and is more likely to irritate nerve roots. The irritation can be from pushing on the nerve or, much more commonly, the herniation causes a painful inflammation of the nerve root.

You can have a bulging disk or herniated disk without any symptoms. Some people only learn they have these disk changes after having an imaging test, such as an MRI, for another medical issue.