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 802.15.4 standard. Its low power consumption limits transmission distances to 10–100 meters line-of-sight, depending on power output and environmental characteristics ... Wikipedia
XBee is the brand name of a family of form factor compatible radio modules from Digi International. Wikipedia