Difference between US QWERTY and International QWERTY Apple keyboards?
The most important differences are the size and position of the Enter/Return, and a few differences in the layout. The ~ key and the \ key have an entirely different position on both keyboards. Also, the US keyboard has no € label (although it can be entered: Alt+Shift+2). 

The International keyboard (‘keyboard type’ = ISO) also has one more key than the US keyboard (‘keyboard type’ = ANSI).  Some Apple keyboard layouts use that key for essential characters, and this can cause problems for users with only the US keyboard.

Below is a high-quality visual comparison.

US QWERTY Apple keyboard:



EN International Apple keyboard: