If I recall correctly, the MacCharlie was essentially an IBM PC clone in a beige box that hooked to the Mac's serial port. As a result, the Mac merely served as a serial terminal for the MacCharlie via custom terminal software running on the Mac. That's not a particularly efficient setup, but the lack of expansion ports on the original Macintosh meant that there was no other reasonable point of entry.
Since it worked through the serial port, the MacCharlie could only run text-based MS-DOS applications. Conveniently, the MacCharlie shipped with a keyboard extender that added the IBM PC's special function keys and a numeric keypad to the Macintosh keyboard.
Discussion Topic of the Week: Have you ever used a hardware system adapter (something that lets you use software from one platform on another through hardware, not software emulation) for any computer system?