Far-seeker:
Transistors can be used in two main ways that are analogous to more common physical devices where a smaller signal (either voltage or current) controls a larger amount of voltage or current. The first is like a valve, were the smaller signal controls how much of a larger signal is allowed into the rest of the circuit. The second is like an on/off switch, where the presence of the small signal determines if the larger signal is completely blocked or not.A "MOSFET switch" refers to a MOSFET, a type of transistor that is controled by voltage levels, configured to be such a switch.
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.
Please correct me if I'm wrong, or if there are alternate setups.
Also, I get the vibe that it's best not to run everything's power through the Arduino.