Tire load ratings define how much weight each tire can safely support when properly mounted, balanced and inflated. These ratings are arrived at by extensive testing performed by the Department of Transportation and the tire makers. Testing consists of tire samples being mounted on proper wheels and calibrated weight being applied until the tire fails. This failure weight is recorded, and a percentage is removed as a safety factor. As an example, if a particular passenger car tire is tested and fails at 1,300 pounds, the tire maker will rate that tire with an 89 load rating, which means the tire can safely support up to 1,279 pounds. Passenger tire load ratings can be found here. Light truck tire ratings can be found here.
For more info, see 3 Tire Care Secrets of the Pros