Contractor is one that provides goods or service to costumers. The contractor can be a group or individual that has a contracts with another individual or organization to make a project or something especific. The contract agreement between the customer and the contractor can include everything from the responsability for the methods, means, execution and/or supervision to the supplying of all material, labor, equipment and/or services necessary and it's include the budget, the general and special conditions and the plans and specification of the project, service or supplies.
To do this it is common for the general contractor to subcontract part of the work to other persons and companies that specialize in these types of work. These are called subcontractors.