This declared two variables 'a' and 'b'. Because they are contained within the 'main' function, they are local to it. If we create 'a' or 'b' somewhere else, they would be referring to different variables. We are calling the 'printf' function. This function is declared in the file and is defined within the operating system.
All functions can reference myglobalvariable, since it is at a fixed location in memory.
This is the definition for the 'myfunction' we declared earlier. We have to declare and define a function before we use it.