In both systems, the hopper where you place banknotes to be counted is located on top of the machine.
The difference is that in a front-loader, you place the stack of bills horizontally (flat); in a back-loader, you place the stack vertically (on its side).
This difference is also what lets you keep adding bills to the stack as a front-loading machine runs, which can increase your processing speed if you have a lot of bills to count.
