The Sprint Backlog is a selected subset of the Product Backlog.

It contains the stories selected to be accomplished within this Sprint.

The Selection takes place in the Sprint Planning Meeting.

Based in that the Developement Team derives tasks for each User Story and assigns it to a developer.

All work of the Development Team within a Sprint is based on the Sprint Backlog.

The Development Team is responsible for the accomplishment of the Sprint Backlogs’ items.