The first thing you need to know is what the codes stand for. When you see a piece of code 83 rail you are looking at track that has a rail height of .083", code 100 is .100" high. Code 83 is probably the most popular in the hobby due to the fact that it is the closest to that used by prototype railroads.
Can you use them together?
Some layouts will make use of different codes of rail. For example you could use code 83 rail on your main lines and code 70 for sidings and spurs, this simulates the lighter rail used on prototypes. However you will need to use transition joiners when making the switch to a different code of rail.
Also not all rolling stock is compatible with all types of rail. The wheel flanges on some older rolling stock are larger and do not allow the wheel to seat down on the rail properly. Most new rolling stock of today runs best on code 83 track.