A non-contact voltage tester is a safe electrical tool that checks for electrical power on the hot wire by detecting the electrical field emitted by the hot wire.  This is used by almost everyone that does any electrical work to check that there is not electrical voltage before doing any electrical work or indicates that the person should use tools with a non-conductive outer protective skin to avoid getting shocked or electrocuted  The tester is turned on by pressing an on/off switch and many if not most will turn themselves off automatically if a certain amount of .  time has transpired and the unit has not been utilized.  

Another important usage of the non-contact tester is to trace electricity in a wire to the point where the electrical continuity is broken.  I once had a great and expensive microware oven that stopped working.  I took off the back of the unit, plugged it in, and traced the wire from the wall to a fuse which was blown and fixed it for pennies.  And it was easy.  A few month later this happened again and I traced it to the fuse but the fuse was fine.  I continued the trace until I found a blown thermister and again replace the broken thermister for cheap and in the end I got over 39 years of use out of that microwave.

Finally, it should be noted that after turning on the non-contact electrical tester one should confirm that it is working properly by putting the tester next to an electrical cord of a properly functioning electrical device to confirm that the tester is working correctly before relying on it in an active electrical repair.  Note that the tester should indicate power by either emitting a sound or some kind for light to indicate there is active power.

In conclusion non-contact electrical testers are only a few dollars and can easily pay for themselves in safety and performing a very important electrical safety or diagnosis work.