A Burndown Chart is a tool used by multiple software engineering methods to track the progress of work completed.  It compares the amount of work remaining (typically measured along the vertical axis) against time (measured along the horizontal axis).  The amount of work remaining can be measured in whatever way works best for the project, i.e., work-hours, work-days, story points, or any other work unit. Similarly the time axis can be measured using a variety of units, the most common being days or iterations.  The burndown chart gives a quick view of the amount of work that is completed over time. 

When applied to Agile methods such as Scrum, this tool can be used to track progress at the Sprint level (a specific iteration of development) or at the release level (multiple iterations that deliver the total functionality for a product release). After the amount of work completed has been measured over several units of time, the burndown chart can be used to forecast the completion of an overall release or project.