A statement of work (SOW) is a sub-agreement under a master services agreement (MSA). The MSA defines the legal framework (payment, IP, liability); the SOW defines the deal (what's being delivered, by when, for how much).
Good SOWs include: objectives, scope (inclusions and exclusions), deliverables with acceptance criteria, milestones with payment triggers, project team, and assumptions. They should reference the parent MSA explicitly so terms inherit cleanly.