I really haven't played around with it much, but basically i++ should increment a variable after using, while ++i will increment before it is used.
For example if i = 10, System.out.println(++i); would print out 11, while System.out.println(i++) would print out 10. In both cases i would be 11 after the print statement.
At least I think that is correct. I pretty much never write code that it would make a difference.