Battery-powered smoke detectors are stand-alone units. But if you have AC-powered smoke detectors in your home and your home has been built in the last 10 years in the U.S., chances are they are wired together to intercommunicate. This sort of wiring guarantees that if one alarm in the house goes off, they all go off. Even if the fire starts and is detected in the basement, people asleep upstairs will hear the alarm because of this safety feature -- every alarm in the house goes off.
If you buy an AC-powered smoke detector today, it will have three wires -- black, white and red. Black accepts 120 volts AC, white is neutral, and red is the intercommunication wire. All of the alarms operate off the same circuit from the fuse box and are normally connected using normal wire for three-way switches (see How Three-Way Switches Work for details -- this wiring contains black, white and red wires in a Romex casing). The electrician runs the red wire from alarm to alarm to interconnect them.
When any alarm detects a fire, it sends a 9-volt signal on the red wire. Any alarm that detects a 9-volt signal on the red wire will begin sounding its alarm immediately. Most alarms can handle about a dozen units intercommunicating on the same red wire. It's a very simple and a very effective system.