Networks are complex entities that require the interaction of both hardware and software components to transport data from device to device. Networks are typically broken down into layers to break down the complexity into smaller pieces and help aid in network design. The layers within a network are responsible to perform a certain task and also to communicate with the neighboring layers in the network stack.
The ZigBee wireless standard defines several different layers in its network architecture including the PHY (physical layer), the MAC (medium access control), and network, security and application interface layers. 802.15.4 is an IEEE based standard that includes only the PHY and the MAC layers.
The XBee and XBee-PRO products from Digi are currently shipping with the 802.15.4 standard. These modules and modems are capable of supporting the full ZigBee stack, and all modules that are purchased today will be firmware upgradeable to the full stack when it is released.