A subcontractor is an individual or in many cases a business that signs a contract to perform part or all of the obligations of another's contract.
A subcontractor is hired by a general or prime contractor to perform a specific task as part of the overall project.
The incentive to hire subcontractors is a cut in cost to the general contractor while receiving the same or better service than the general contractor could have provided by itself. Most subcontractors do work for the same companies rather than different ones. This allows subcontractors to further specialize their skills.