++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
int main(int argc, char **argp)
int x = 5;
The output of this program would be
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.