What is a stud finder?

The usual stud finder has a short range metal detector that's powered by a battery. You simply press the switch on the side of the unit and move it side to side on the drywall. It will flash (some beep as well) when you pass the detector over a nail head or anything metal. (Nails are usually nailed into studs.) It's a great tool to have in your box, and you don't need to spend a lot of money for a good quality product.