What is a Sprint Backlog?
The sprint backlog is the developers' plan for the sprint: the sprint goal, the product backlog items selected, and the emerging task-level plan to deliver them. It belongs to the developers alone — they shape it daily as reality reports in.
Boundary with its parent: the product backlog is the PO's ordered wish-list for the product; the sprint backlog is the team's working plan for the next two weeks. Mid-sprint additions from outside are how sprints die.
Worked example
Monday's sprint backlog: goal "customers can pay by invoice," five stories, eighteen tasks on the board. Wednesday, the team discovers the tax-calculation task hides three tasks — the sprint backlog grows honestly, one story gets renegotiated with the PO, and the goal survives. The artifact flexed; the commitment didn't.