Permanent intercoms installed in buildings are generally composed of fixed microphone/speaker units which connect to a central control panel by wires. A small home intercom might connect a few rooms in a house. Larger systems might connect all of the rooms in a school or hospital to a central office. Intercoms in larger buildings often function as public address systems, capable of broadcasting announcements.

In many schools, tones signaling the change of classes are sounded over the intercom, taking the place of the electromechanical bells used in older schools. Additionally many schools now use audio / video intercoms to identify visitors trying to gain access to a locked school building. Many intercom systems can be interfaced with the building's access control system.

Intercom systems can be found on many types of vehicles including trains, watercraft, aircraft and armoured fighting vehicles.

Portable intercoms are commonly used by special event production crews and professional sports teams. Performing arts venues such as theaters and concert halls often have a combination of permanently mounted and portable intercom elements. Motorsports race tracks often have both portable and permanent intercom stations mounted at critical points around the racecourse for use by race officials and emergency medical technicians.

Traditional intercom systems are composed entirely of analogue electronics components but many new features and interfacing options can be accomplished with new intercom systems based on digital connections. Video signals can be interlaced with the more familiar audio signals. Digital intercom stations can be connected using Cat 5 cable and can even use existing computer networks as a means of interfacing distant parties.

Basic terms

• Master Station or Base Station - These are units that can control the system, i.e., initiate a call with any of the stations and make announcements over the whole system.
• Sub-station - Units that are capable of only initiating a call with a Master Station but not capable of initiating calls with any other stations (sometimes called slave units).
• Door Station - Like sub-stations, these units are only capable of initiating a call to a Master Station. They are typically weather-proof.
• Intercom Station - Full-featured remote unit that is capable of initiating and receiving party-line conversation, individual conversation and signalling. May be rack-mounted, wall-mounted or portable.
• Wall Mount Station - fixed-position intercom station with built-in loudspeaker. May have flush-mounted microphone, hand-held push to talk microphone or telephone-style handset.
• Belt Pack - portable intercom station worn on the belt. Requires a headset or handset.
• Handset - permanent or portable telephone-style connection to an intercom station. Holds both an earpiece and a push to talk microphone.
• Headset - portable intercom connection from a belt pack to one or both ears via headphones with integrated microphone on a boom arm. Connects to a belt pack.
• Power Supply - Used to feed power to all units. Often incorporated into the design of the base station.