This is a way to re-composite a render from his render passes. There is no direct application because this tutorial wants to retrieve the orginal render but it mays help you to get further with passes and compositing.
The official documentation on Passes (release log of 2.43) tells you a bit about each render pass.
You can download this test scene with the nodes above.
Troubleshooting / Improvements to do
- AO is not well composited.
- Does'nt work with transparency/reflection materials.