The most important thing to understand is which frequencies, or bands, the phone supports. GSM carriers in the United States, Canada, and many parts of Latin America rely on the 1900 and 850 bands for transmitting cellular phone calls and data.
In the United States, the 1900 band is used extensively in urban and developed areas, while 850 has largely been used to boost signal strength in outlying areas. At the minimum you'll want a phone that supports the 1900 band if you live in North America.
Europe and many other parts of the world employ two other bands: 900 and 1800. If you plan on traveling a lot internationally but want to own only one phone, consider purchasing a quad-band phone, which will give you coverage all over the world.
If you aren't a frequent international traveler but do make occasional trips, a less expensive tri-band phone will probably suffice. A phone that supports the 1900, 850, and 1800 bands will provide great service in the United States and passable service in Europe.
The other thing to keep in mind is that even though unlocked phones do offer some advantages, they only work with GSM-based T-Mobile and AT&T networks. If you prefer Verizon or Sprint, you might want to stick with their pre-configured phones.
