++x is pre-increment and x++ is post-increment that is in the first x is incremented before being used and in the second x is incremented after being used.
This sample program might be of use here
[insert]
Code:
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char **argp)
{
int x = 5;
printf("x=%d\n", ++x);
printf("x=%d\n", x++);
printf("x=%d\n", x);
return EXIT_SUCCESS;
}
The output of this program would be
6
6
7
So you see in the first case x is incremented before being printed to the console
In the second case the value of x is incremented after the value is printed to the console
and in the third the result of post increment in the previous printf can be seen.
Hope this makes it a bit clearer.