In the ScrumProcess, this is the prioritized list of all features and changes that have yet to be made to the system.  One way to think of it is as a project-level ToDoList.  Another way is to view at as "potential software."  A third way is to see it as one of the raw materials fed into the ScrumProcess.

The ProductOwner is the ultimate arbiter of what's on the list, and what the priorities are.  OneBossPlease.

At the beginning of each 30-day ScrumSprint, the ScrumTeam and the ProductOwner agree on a selection of items from the list that will make up the SprintGoal.  
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See: SprintBacklog
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CategoryScrum