We receive a lot of questions about what a contact mic is and how to use it, and so here is a quick overview.
A contact microphone is used for picking up vibrational sounds. Contact mics employ a type of element called a "piezo electric transducer", which consists of a crystal mounted to a brass, copper, or steel disc. When the disc receives a vibration its piezo crystal converts this into an electric signal which is sent to an amplifier in order to be audible. The best way to understand a contact mic is to think of it as an electric stethoscope! As such, it can be a very effective tool for amplifying acoustic instruments, making interesting field recordings, and doing the deep dive into experimental sound design.
Contact microphones are passive transducers and do require a preamp. A preamp can be anything that boosts the signal strength (aka, impedance) to line level. A guitar amplifier and effects pedals will do the trick, as well as mic/line mixing boards, microphone interfaces for computers, and nearly all portable recorders featuring external microphone jacks. 