What is the difference between the two types of int

Question

You declare a program with
int main()
But you can define a variable as an integer
int a
What's the difference

Answer 1

main() is a *function* of return type int in which your program is executed.
a (as in your example) is an integer *variable*.

Answer 2

int main just means that the return type of the whole program is in integer, hence why return 0; is at the end of all programs