The sprint backlog is the team’s to do list for the sprint. Unlike the product backlog, it has a finite life-span: the length of the current sprint. It includes: all the stories that the team has committed to delivering this sprint and their associated tasks. Stories are deliverables, and can be thought of as units of value. Tasks are things that must be done, in order to deliver the stories, and so tasks can be thought of as units of work. A story is something a team delivers; a task is a bit of work that a person does. Each story will normally require many tasks.
