Plots units of work that remain to accomplish (Y axis) against units of time (X axis). In Scrum, the Burn Down Chart is a key artifact.
Release Burn Down
The units of work that appear on a Burn Down Chart are derived from the Release Backlog. During the process of Backlog Grooming they have been assigned an estimated point value. The trend line on a Release Burn Down Chart will generally trend downward, however, if new items are added to the Release Backlog, then the total points remaining may go up.
The Release Burn Down Chart is the primary tool a team has for visualizing their velocity, the average number of points they accomplish during an iteration.
Iteration Burn Down
The initial point value of work remaining in a Sprint Burn Down chart derives from the work the team commits to accomplish during the Sprint. Work remaining is generally graphed daily. The number of points the team undertakes is based on their established team velocity, i.e., the number of points they routinely complete. In Scrum, no new work may be added once a sprint has begun, so the trend line will never rise. In Extreme Programming, work may be added during a sprint, so the trend line may rise.