In the UK and Europe, GSM is operated on two frequency ranges, 900MHz and 1800MHz. The US uses 1900MHz. Tri-band phones can use all these frequencies.
Whereabouts in the world are you, Salmonaxe, since spectrum plans are defined by national governments and vary throughout the world.
Generally:
HF - 3-30MHz
VHF - 30-300MHz
UHF - 300MHz-3GHz
SHF - 3-30GHz (also called millimetre wave)
EHF - 30-300GHz
Regards
Dave Roseman