A driveway alarm usually consists 2 components: Transmitter and receiver. The transmitter is places where your driveway starts (of course also depending on the range of the system you get) when someone passes by the transmitter (also referred to as sensor) the receiver will get a signal. Some system have wireless receivers which means you can get an alert of trespassing for example pretty much wherever you are and some are hard wired and you will only get the alert from the transmitter if you are physically home.