There are two types of disability insurance: short-term and long-term. Most employers pay for short-term disability as part of the benefits package. While they also offer long-term disability, the premiums generally come out of your paycheck.
Long-term disability insurance is designed to make up part of lost income if you are unable to work for health reasons. The health issues need not to have arisen from work, but they must be severe enough to render it impossible for you to continue in your present employment.
Long-term disability usually kicks in after the short-term policy has expired. If you qualify for social security disability, you may have to pay a portion or all of the money received back to the insurance company. The policy covers a portion of your salary, but not all of it.