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.

Disks show signs of wear and tear with age. Over time, disks dehydrate and their cartilage stiffens. These changes can cause the outer layer of the disk to bulge out fairly evenly all the way around its circumference — so it looks a little like a hamburger that's too big for its bun.

A bulging disk doesn't always affect the entire perimeter of a disk, but at least a quarter if not half of the disk's circumference is usually affected. 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 protrude out of the disk. Herniated disks are also called ruptured disks or slipped disks, although 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 because it generally protrudes farther and is more likely to irritate nerve roots. The irritation can be from compression of the nerve or, much more commonly, the herniation causes a painful inflammation of the nerve root.

If an imaging test indicates that you have a herniated disk, that disk might not be the cause of your back pain. Many people have MRI evidence of herniated disks and have no back pain at all.