The Xbee OEM RF modems are an accessible low-cost wireless data transfer solution.
An Xbee module can be connected to an embedded system such as the Arduino boards www.arduino.cc/ or to any computer system via a UART interface. en.wikipedia.org/wiki/UART
More and more the Xbee is becoming increasingly usable as new development boards and USB interfaces become available. http://www.dontronics-shop.com/ At first the technology can seem a little like black magic, but it is easy to grasp at least a simple understanding from reading a little of the literature available. The summaries that I have included here are to do with operation of the Xbee modules using the different topology and communication mode than that which is set as default.
More often than not the default operation may be sufficient for projects, I have generally used the default mode myself, but some applications could benefit from the other modes of operation, especially as the use of the technology becomes more prevalent.