extern

Can anyone explain me what extern is used for? I thought it was used to declare variables definited in other files, but i can do that also without extern.

/*file a.c*/
int a=5;
int main()
{
f();
}

/*file b.c*/
int a;
void f()
{
printf("%d\n",a);  /*it prints 5*/
}


Another thing, what happens if i declare a function like this: extern int fn(void) ?

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