According to the Scrum Guide, traditional Sprint Burndown charts measure:
The amount of work remaining for a Sprint is the sum of the work remaining for all of Sprint Backlog. Keep track of these sums by day and use them to create a graph that shows the work remaining over time. … Duration is not considered in Scrum. Work remaining and date are the only variables of interest.
Traditionally people have been taught to measure the remaining work in hours. As Mark Woyna points out when tracking task hours it's possible to burn down many hours without completing any stories. In that case the burndown is hiding the real progress of the team. He goes on to suggest that many teams track story points completed. When Adam Sroka sees this problem is usually because the team has started working on too many things at the same time.
This reporter notes that tracking task hours encourages us to complete tasks whether they’re still relevant to the completion of a story, encouraging clients not to estimate tasks in hours and not to use a burndown chart. Instead we watch the progress of stories and tasks across the team’s sprint board.
Johanna Rothman measures: Stories completed, stories remaining and total stories, both at the sprint and release level.
Aslak Hellesøy recommends Culmulative Flow Diagrams over Burndowns, noting that they provide more information.
Do you still use Sprint Burndown Charts? If not what have you replaced them with?