A test light is normally used as a basic circuit testing tool to ensure that the circuit is carrying a voltage to enable an electrical component to operate. It is normally constructed in such a way that the tester is insulated from the user and has a small bulb in the circuit. One end of the tester has a "spike" probe that is able to pierce the insulation of cables and the other end has a small clip that is able to be clamped to a suitable return point for completion of the circuit.
Source: CDX Global & Wikipedia - en.wikipedia.org
