a never ending task list which, if you finish, the (user) story says a bright white light will appear. Don`t walk into that light. On a more profane note, the backlog is a list of new features, changes to existing features, bug fixes or any other development tasks that need to get done within a project. The backlog feeds the sprints. The team selects stories out of the backlog during sprint planning and turns them into the sprint backlog. Nothing gets done if it's not in the backlog. Alternatively, nothing gets done anyway.
“Getting an office dog and calling him Backlog was the best project management decision we`ve ever made. Our PMs now groom it daily.”