Function or procedure?
 

What are the mains differences between these and what's the better indipendently their usage

When I think about functions and procedures, Pascal comes to my mind. In C, there are only functions.

It's simple (in Pascal): procedure is a function that doesn't return anything. That's the only difference. When you write something and you need to return a result, you use a function. If you don't return anything - it's procedure. No benefits.