Newbie gcc question

["Wayne Willson" <wmw_will@yahoo.com>]

I'm a sys/network admin trying to learn C (this could be the root of the
problem) with the GNU C compiler.

In a learning C book I just bought it gives this example

--------------------------------------------
#include <stdio.h>

viod main()
{
        printf("Hello Universe");
}



I type "gcc -o hello hello.c"

This is what I get "hello.c: In function `main':
hello.c:6: warning: return type of `main' is not `int'"
------------------------------------------------------

When I try this it works:
#include <stdio.h>

int main()
{
        printf("Hello Universe");
}

------------------------------------------------

This works as well

#include <stdio.h>

main()
{
        printf("Hello Universe");
}

-------------------------------------

Could someone tell me why gcc doesn't work with 'void'  but works with 'int'
and '   ' ?

Thanks