An IoT device is the physical unit at the edge of an IoT system. At minimum it has something to measure or control, a processor running firmware, a power source and a radio or wired interface. The processor may be a tiny microcontroller waking a few times a day, or a full application processor running Linux and a camera pipeline.
Those two ends of the spectrum behave very differently. A battery-powered leak sensor must spend most of its life asleep and send a few bytes when something changes; a connected security camera has mains power and streams video continuously. Cost, power budget, memory and the quality of the network all shape the design, which is why a single product line often ships several device classes. The guide on IoT device types and constraints walks through them.
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