An intercom system is a two-way communication electronic device that contains circuitry for the purpose of transmitting and receiving audio and/or video transmissions.
With the advent of cellular communication, digital telephone systems, and other communication devices, the use of intercoms has declined. However, there are still lots of applications where these devices still work better than anything else. There are a very wide variety of systems available as defined below:
Video Intercom: These typically have a unit that has a camera, speaker, and a push button that goes at an entrance door, and an internal monitor unit that can communicate with the entrance unit. Some systems can have multiple video monitors internally. Given the difficulty of sending video over the airwaves at very much distance without interference, most available units are wired.
Apartment Intercoms: This type of system has a door unit at an entrance door and then each apartment or flat has either one or multiple units that can speak to the door and possibly see the visitor, or the door unit rings a landline or cellular telephone number. A visitor simply presses the correct button for the apartment they want to speak to, and the tenant can speak back and then press a button to unlock the door.