<< What is GSM? >>
GSM is the global standard for mobile telephones. It's the dominant technology in Europe (operating at 900 and 1800MHz) and Asia. There are operators in USA that use GSM (at 1900MHz), but it's not dominant there.
THe good thing of single standard, is the fact that I can take my cell-phone (Nokia 7110) ad go from Helsinki to London, to Madrid, to Rome, to Paris, and back to Helsinki again... And during that time my phone would work just as it did before.