A continuity tester is a device that is powered by batteries and has a probe at one end and a cord with either an alligator clip or another probe at the other end. If you touch the two together, you complete a circuit and light is illuminated on the body of the tester, indicating a complete circuit. These testers are great for checking to see if something like a single-pole switch is working properly.

Unlike other testers, a continuity tester is always used when a circuit is turned off or on wiring or devices that are disconnected from the circuit. If you are using a continuity on a device attached to circuit wiring, always turn off the power to the circuit or device that you're be testing. Or, disconnect the device entirely from the circuit wiring. It can be very dangerous to use a continuity tester on wiring that is carrying voltage. 

Some electricians only use these tools to test devices that are entirely disconnected from circuit wiring. For example, the continuity tester offers a good way to see if lamp wiring is intact or if the inner mechanism on a disconnected switch is functioning properly. If a circuit has been turned off, a continuity tester also provides a method for checking wire runs to make sure that a circuit is complete. You can also use them to identify short circuits in wiring.