A device could work entirely on its own, but exchanging with other devices and internet is quite cool, isn't it ?
In the home automation world, a device is an autonomous module. It often takes input, via sensors (smoke, button), to compute a result. A smoke detector, for instance, detects smoke and produce/compute things, which will be sent (or not) to one or more other devices. A device could work entirely on its own, but exchanging with other devices and internet is quite cool, isn't it ? This is what we call IoT (Internet Of Things). Each device in our lives, could potentially become an internet connected device, capable of computing things, and exchanging informations regarding its sensors. The three main protocols used for wireless communication are Wifi, Z-Wave and ZigBee. Each one have specificities and features.