CSEngineer13:
Ahhh. So a MOSFET switch would be ideal IF the device were powered by the Arduino, but not if it were powered by a separate source.
For example, the Outlet -> Power Tail -> AC Adapter; where the Arduino tells the Power Tail to cut the power to the device.
Actually a primary reason to use a transistor switch with an Arduino is when you don't want to power some component from the Arduino board, but you still want the Arduino to control DC power to this component (for AC it's often better to use a device called a TRIAC). The digital I/O pin is providing the small control signal to the MOSFET and that will determine if power is allowed past the transistor. It's most intuative and, with the proper type of transistor, easiest to setup the circuit so when an Arduino I/O pin is HIGH the transistor is open or "on".