Statement of Work
A description of the products, services, or results to be supplied to a customer. It will usually include the expected project cost, terms and conditions including payment terms and change control procedures.
Typically it will specify if the customer will pay a fixed cost or time and materials. It may also include a payment schedule specifying how and when payments will be made. For example, the supplier and customer may agree to payments on completion of certain milestones (design sign-off, user acceptance testing complete, go live and so on).
SOWs are often the next step following a successful Request for Proposal (RFP) and they may refer to and be supported by a separate Master Services Agreement (MSA).