From Wikipedia UMTS: Universal Mobile Telecommunications System (UMTS)' is a mobile standard of the third-generation (3G).
In Japan, only UMTS is available, no GSM network exists there. For this and other reasons, support for UMTS is very desirable. This can be accommodated in the device itself or externally with supported peripheral hardware via UMTS tether mode using USB, Bluetooh, WiFi or infrared. The 2nd generation of Apple's iPhone (iPhone 3G) supports now UMTS, too.