A statement of work (SOW) is a document that spells out the work to be performed. PMI describes two kinds of SOW: project and procurement.
A project statement of work describes the product(s), service(s), or result(s) to be delivered by the project. It is an input to the PMI process to develop project charter. It documents the business need, scope, and the project's alignment to the strategic plan. See section 18.104.22.168 of the 5th edition of the PMBOK.
A procurement statement of work describes the work to be completed as part of a contract. It may cover quantity, quality, timing, methods, materials, or other criteria. See section 22.214.171.124 in the PMBOK 5th edition.