Hmm... I could pop in with my photographer courses and stint in printing and such. I'll give a small crash course I guess...
Aperture - As discussed, this will affect the Deep of Field. In real life, it will also affect your exposure, but I don't think Forza gives a crap, allowing you to have unreal settings with the quality of "film." Plus side is you can shut it down all the way to maximize DoF, and still shoot with a fast shutter speed. In regards to how a pro would use this, in normal situations, would be to open it up enough to allow the subject to be all in focus, while keeping excessively distracting BG's from being out of focus.
Shutter Speed - Adjusting this will affect blur and can be very useful in conveying a sense of speed. Too fast a shutter speed will result in still wheels, which makes the car look like its standing still at 100 MPH. You can set it to where just the wheels start to blur, etc. Its also import if point of focus is an issue.
Focus - Not sure on how Forza handles this as of yet, but in GT4 this is how it worked. Focusing on the car would simulate tracking with it, which leads to the motion blur on the back ground, while keeping the car sharp (Wheels will blur from spinning). However, focusing on the track next to the car will give it the motion blur. It will also affect where the DoF sits with the picture, allowing you to have the back of the car out of focus, with the front sharp (with a wide open aperture).
Exposure - How much light gets on the film in real life, here just the brightness and saturations. In real life, exposure is also determined by shutter speed, aperture, lenses size, and ISO. The game makes it much easier
Quick question. Does Forza 2 offer a filter option, such as using a polarizing lens? If so, that could be used to generate truly awesome pictures. I'll see if I can find a good example of what can be done with a polarizing filter.