The sprint backlog is very commonly maintained as an Excel spreadsheet but it is also possible to use your defect tracking system or any of a number of software products designed specifically for Scrum or agile. An example of the Sprint Backlog in Excel looks like this:
During the Sprint the ScrumMaster maintains the sprint backlog by updating it to reflect which tasks are completed and how long the team thinks it will take to complete those that are not yet done. The estimated work remaining in the sprint is calculated daily and graphed, resulting in a Sprint Burndown Chart