Cumulative Flow Diagrams are a wonderful tool to see trends and find bottlenecks in your delivery process. They are often used in Agile environments.
Consider the example of a Website Development Project below. You will see a graph below. It shows the number of user stories in each of you status categories, for the time period you have selected. If you draw a horizontal line at any point on this graph you will see a snapshot of your User Stories on a given date (how many with status “Done,” “TO-DO,” “Testiing,” etc.).
So the CFD is really just a picture of your user stories by status over time. But this picture can tell you a lot about your development process.