Unlicensed Mobile Access (UMA) is gearing up to be the "next big thing" in wireless communications. It’s the ‘Reese’s Peanut Butter cup’ of the mobile phone industry. You know: "You got your WiFi in my cell phone; you got your cell phone is my WiFi". Actually, maybe you don’t know what UMA is: UMA phones are WiFi-enabled cellular phones that are essentially smart enough to use a cellular network when they have to, but they seamlessly switch your call to a WiFi network when they can. The transparent network switch allows for lower costs and potentially more coverage, depending on the WiFi mesh during your travels.
Mobile Crunch has a detailed primer, with plenty of techie terms and diagrams; probably worth a read now that you have the bare-bones basics.