Nikon FM2 back

The back of the camera holds the film against the film plane and provides the camera interior with absolute darkness, no light. If there were any light entering the film place the film would become esposed and damage the film.
The square box on the back is to tear off the flap on the film box and to insert, this tells you the film ASA/ISO speed and the brand and kind of film, when you change film to another type or speed change this out also.

Film ASA/ISO

ASA and ISO basicly are the same thing
It is a standard all film makers follow for speed or sensitivity of film
Kodak is not the only brand of film made, there is Fuji, Ilford and others
there speeds differ and so dose their use.
The internet can provide you lots of details about 35mm film and what each film does best, worst, and what the film type was produced for

A friend of mine used to get Hollywood Film called 5247 which is what they used commerically and it could be used with different speeds on the same film.
He seemed to like it and got different results from it.

I purchased my first 35 SLR camera because of this friend, Alan Rich, he loaned me his Pentax MX camera for a trip I went on to Missouri, and the results I got were unreal.
I liked that camera so well I purchased one exactly like it, last summer my son used the camera and got great results still.

I got the Nikon because I like the large bright viewfinder and the faster 1/250 th of a second on the flash.
I purchased the Nikon for about $119 dollars on Ebay, so even a great camera can be had for a decen price.
Digital is the new way to do things, but so far none can match the brilliance, clarity and texture of the film.
Digital is coming a long ways now and the high end digitals are SLR's just like the film cameras. One day they will approach film and they are cheaper because of no film purchase or development.
However you still can't be as creative with a digital as you can be a film camera at this point.