XBee is the brand name from Digi International for a family of form factor compatible radio modules. The first XBee radios were introduced under the MaxStream brand in 2005 and were based on the 802.15.4-2003 standard designed for point-to-point and point-to-multipoint communications at over-the-air baud rates of 250 kbit/s.

More information on XBee can be found on this Wikipedia article, where there also are many useful links (the excerpt was also taken from it).