Wireless LAN Media Access Control and Physical Layer specification. 802.11a,b,g,etc. are amendments to the original 802.11 standard. Products that implement 802.11 standards must pass tests and are referred to as "Wi-Fi certified."
ZigBee is a specification for a suite of high-level communication protocols used to create personal area networks built from small, low-power digital radios. ZigBee is based on an IEEE 802.15 standard. The technology defined by the ZigBee specification is intended to be simpler and less expensive than other wireless personal area networks (WPANs), such as Bluetooth or Wi-Fi.
Operating at 900 MHz yields significantly longer range (approximately 2 times) than what is possible at 2.4 GHz. The 900 MHz waves penetrate objects and travel better than their 2.4 GHz counterparts. If you are in the US, Canada, or Australia you will be better off range-wise using the 900 MHz radios.