This is why my car was in the shop last October.
These are both the same picture, I just doodled on one.
This is a picture that shows the various components involved in the work being done. This picture was taken from the front of the car, facing toward the back. This is what you see if you remove the front bumper, part of the frame, and the radiator. The timing belt (already removed) connects the two large wheels – camshafts – on the top-left and top-right of this image to the small rotating shaft – crankshaft – in the center of the image.
The crankshaft translates linear piston motion to rotation, as shown below.
(The crankshaft is the bottom left of this picture).
The crankshaft rotation drives the timing belt, and the timing belt drives both camshafts. As the camshaft rotates, oblong cams along the length force open and close the valves — my Passat has 30 valves, 15 on each side.
If the timing belt breaks, the valves are no longer synchronized. As a result, the pistons can fire straight into the valves and ruin the engine. This only needs to happen once, so if you don’t hear the timing belt break while driving and keep the engine running for even a few seconds, the engine is toast and the car is totaled.
VW says that the timing belt on the Passat is built to last around at 100k miles, and my car has 124k miles. Therefore, the timing belt had to be replaced.
In these pictures, you can see the camshaft a little better — it’s the shaft attached to the wheel with spokes on the top right of the leftmost image. The rightmost image shows the guilty party: the smaller belt leaning against the toolbox is the timing belt. It has deep ridges that fit into the gear teeth tightly, so that there is no slipping. Slipping would throw off the timing, which is no good. Some cars, like BMW, even use a chain for improved durability (and of course, with a chain there can certainly be no slipping), but chains tend to be noisier.
The other (larger) belt in that picture is drive belt, which transfers power in the form of rotary motion to the alternator, the power steering and the AC (a single belt does all three). It was also replaced.
It is not easy to the get access to all this assembly on my car. The whole front of the car must be removed.
The ignition wiring which you can see below is also being replaced ($$$!) — and this is the old water pump, removed.