Back in the bad old days cameras ate a substance called "film." You would feed it a roll, take 12, 24, or 36 photos, extract the roll from the camera's tummy, and send it off to be processed. Some time later you could look at your pictures.
This view shows the winder and frame counter on my old F3. I love that camera, but haven't shot anything with it in a long time.

It looks like this is image number 35,359 on my D300. A 36 shot roll of Fuji Velvia costs $7.00. Processing would add another $10.00, so figure about $0.47 per frame. As mentioned by Cory, electrons are free. It would have cost me over $16,000 to do this on slide film. Wow...
I think I can justify buying myself a D3s now!
Or not.
/rl