A room loop is one type of loop. It is a large loop of wire that can be plugged into an ALS or amplifier, a radio, TV, some computer speakers and some telephones.
It can be permanently installed under the floor, in the ceiling, around a baseboard, or around an area in a room, or it can be temporarily placed around any area where people with hearing aids containing a telecoil might be.
The advantage of a room loop is that it can support as many people with telecoils as can fit within the loop, and all will be able to hear the broadcast audio through their telecoils. Properly designed room loops can support relatively large rooms, but generally are limited to just a portion of very large spaces like auditoriums or theaters.
The room loop generates a magnetic signal that can be "heard" by the telecoil and amplified by the hearing aid.
Other loops can be used to generate the magnetic signal, too:
Another alternative to a hearing aid's microphone as the input device is the Direct Audio Input (DAI).