A truck's towing capacity tells you how much the truck can pull behind it. This capacity includes the weight of the trailer; if you're close to the limit, make sure you remember this. The ability of the truck to pull weight depends on its engine, transmission, and frame. Many heavy-duty trucks can tow over 10,000 pounds.
Payload is different in that it describes how much a truck can carry in its cab and truck bed. This number is determined when the truck's weight is subtracted from the Gross Vehicle Weight Rating.