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. Here is also rumored that Apple will support UMTS with iPhone.