GSM ("Global System for Mobiles" - or something similar in French!) is the international system. I have used my dual-band mobile in Europe, Mid-East, Singapore and Aussie.
CDMA, PCS is N.America's "as usual, we're different" local system (like NTSC vs. PAL
N. America also uses GSM these days - but as one would expect, the use a different frequency band (
) "The Rest of the World" uses 900MHz and 1800MHz but the DammYanks decided to use 1900! This is why you need a "tri-band" if you want to use the GSM there.
Now I got myself a Motorola Timeport, which is a "tri-band" - which was delivered to me the day after
I got back from my last USA trip (oh well!)
Your coverage, world-wide, will depend on your service provider and who they "partner with" overseas (so check before you make a commitment!) and you will pay a pound or two more per month to have "roaming".
If you look at the coverage map (from your service provider) of North America, it initially looks as though there is not much cover - until you look in detail and see that most of the bits not covered are the bits where only cows live! (and my daughter lives in Philly, with a phone from Sprint, and went to complain that she is always "out of coverage" only to find that the Sprint office in the middle of Philly was "out of coverage!!!)
My provider's (I use one2one in UK) partners (6 different ones in USA) cover most of the E. Coast cities from BOS down to South East, the major cities throughout USA plus the major cities in Canada (at least along the North side of the geat lakes up to Montreal) - I suspect that's as much as you can hope for) Can't quote Korea or Japan, as one2one's "roaming guide" does not include these countries at the moment - maybe another provider has partners there so check which countries you want and which provider gives you the best "basket"!.
What Goes Around . . . . .
. . often makes a better landing