In America phones operate on a dual band system - 850 MHz and 1900 MHz. In the big cities they use 1900MHz, and mainly 850 MHz in outlying areas. If you buy an American phone it will normally be a dual band for those two frequencies, but Tri-band and quad band phones are readily available.
Here in the UK we use 1800MHz or 900MHz, depending on the network. Check that the phone you're buying will operate on those bands, and remember that you may need to have it unlocked before you can use it on your network.