Burndown Chart
A burn down chart is a graphical representation of work left to do versus time. The outstanding work (or backlog) is often on the vertical axis, with time along the horizontal. That is, it is a run chart of outstanding work. It is useful for predicting when all of the work will be completed. It is often used in agile software development methodologies such as Scrum.

sprint burndown
A sample burn down chart for a completed iteration, showing remaining effort and tasks for each of the 21 work days of the 1-month iteration.

The sprint burn down chart is a publicly displayed chart showing remaining work in the sprint backlog. Updated every day, it gives a simple view of the sprint progress. It also provides quick visualizations for reference. There are also other types of burndown, for example the Release Burndown Chart that shows the amount of work left to complete the target commitment for a Product Release (normally spanning through multiple iterations) and the Alternative Release Burndown Chart, which basically does the same, but clearly shows scope changes to Release Content, by resetting the baseline.

We would like to suggest you the following list of usefull resources on the topic: