A statement of work defines the work to be done for a project. It defines all the requirements to client. It includes what must be delivered, how and when it must be completed by. These are all the projects deliverables.
A statement of work is often used between a client and supplier. It should not be a template. It should be custom to the client. It is often a few pages in length and it must be well written, clear, and concise with little ambiguity.
It is the first deliverable the client receives often soon after an initial meeting with the client. It confirms the understanding of what was discussed. It tells a story by giving the background of the situation and what needs to be done to fix it. It is a simple contract. It documents the terms that have been agreed in the initial meeting.