IoT(Internet Of Things)
WPAN - ZigBee
Technically, ZigBee is a kind of an application of IEEE 802.15.4. 802.15.4 defines various types of PHY implementation and a single (common) MAC layer that applies to all the different PHY implementation. In terms of MAC layer, you may say ZigBee complies to 802.15.4 and in terms of PHY layer, you can say ZigBee implements a sub set of 802.15.4 PHY specification. In a very detailed implementation, there is some possibility that ZigBee PHY layer has a little bit of variations from 802.15.4 PHY specification since ZigBee has been in the market for pretty long time and it was commercialized even before every details of 802.15.4 PHY has clearly been defined. In addition, ZigBee has been driven by its own alliance group (ZigBee Alliance) as you might have seen how Bluetooth has been evolved/promoted by Bluetooth Alliance (or Bluetooth Special Interest Group). If you are seriously interested in the technical details of ZigBee, you would need to cover both IEEE 802.15.4 and various documents published by ZigBee Alliance.
Following is overall sturcture of ZigBee in comparison with 6LoWPAN. As you see here, ZigBee uses its own PHY (though it is very similar to 802.15.4 PHY) and the biggest difference is Network Layer. ZigBee uses its own proprietary Network Layer protocl. Since it is Proprietary, it was difficult to communicate to other network(non-ZigBee network), but now it is evolving into more common IP Network Layer concept.