Telecoils
Telecoils (also called induction coils, magnetic induction coils, T-coils, or T-switches) are used, like microphones, as a method of getting sound into the hearing aid to be amplified.  While most people think of the "Telecoil" for use only on the telephone this little device can be used to attach a wide variety of sound sources to the hearing aid. 
The telecoil works on the principle of magnetic induction.  Devices like the telephone receiver (required by law to be hearing aid compatible, meaning the receiver will generate the magnetic field) or a neckloop give off a magnetic field that can be picked up by a specially designed coil of wire and metal.  This pickup coil's output is a electrical charge that can then be amplified by the hearing aid. 
Telecoils are generally built into most behind-the-ear hearing aids sold today.  Telecoils, while included on some of the top-of-the-line model in-the-ear (ITE) and in-the-canal (ITC) hearing aids generally must be ordered as a special option at the time of purchase.