Disability Insurance is designed to protect your greatest asset - your income from your job. If one is not able to work, it's very difficult to provide for a family. Disability insurance protects your income while you must be out of work.
Disability Insurance falls into 2 categories: Short Term Disability & Long Term Disability.
Short Term Disability (STD) policies may last anywhere from 3 months to 2 years. Long Term Disability (LTD) policies generally have a 3 month or 6 month waiting period (you would need to be out of work for 3 months or 6 months before the policy would began paying). But the policy generally lasts until you're 65 or 67 years old.
STD policies are generally only available through a person's job. Sometimes it's paid for by the employer, other times it is a voluntary benefit the employee can purchase as a payroll deduction from the check. LTD policies can be offered to employees through a job or may be purchased as a standalone policy outside of the workplace.
Various insurance companies define "disability" different ways so make SURE you speak with someone who is able to explain the differences and help educate you about your plan.