why we use return 0 in the end of program

Answer 1

Usually it means that program was ended without error. But you also can use main without any return value. (void main)

Answer 2

Martin Taylor i don't say that this is standart....i say, they can use it, cause i don't know about which platform he(she) asked... On operating system - i don't write on C, but in SL Playground, other emulator we can use void main, and most important if we write for MCU, i don't seen IDE which contain int main method, all of this contain void main...