what is 802.15.4 (zigbee) wireless communications?

Answer 1

802.15.4/ZigBee is the intended standard for low-powered networks for use in "smart home" applications like wireless monitoring and control of lights, security alarms, motion sensors, thermostats and smoke detectors (call it a "competitor" to X-10 standard).
ZigBee networks are to run in the unlicensed frequencies, including the 2.4 GHz band.