Zigbee is a complete protocol stack, from radio to application profiles, layered on the IEEE 802.15.4 physical and MAC layers, mostly in the 2.4 GHz band. Devices form a mesh network: mains-powered routers such as bulbs and plugs relay traffic, while battery end devices sleep and wake briefly to report. A coordinator, usually inside a hub, manages the network and bridges it to Wi-Fi or Ethernet.
Its standardized device clusters are what let a bulb from one vendor join a hub from another, which made Zigbee the quiet workhorse of smart lighting and door, motion and climate sensors. The trade-off is that Zigbee does not speak IP natively, so every network needs a hub, and 2.4 GHz coexistence with Wi-Fi can require careful channel choice. See Zigbee explained.
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