Most countries use one or two bands. Most Europe uses 900/1800 bands. Usually a Tri-band will work in most countries. They are either 900/1800/1900 or 850/1800/1900. There is no such thing as a quad-band country. I believe 850 is used only in USA and by Cingular. My 850/1800/1900 band phone worked in UK and Italy. So did my other 900/1800/1900 band phone.
Check the following
Scroll all the way down in my second link and you will see the country list and the bands used.
I'm trying to figure out whether i can use a tri-band phone in a country where they only provide quad-band service. Is it possible to use tri-band for quad-band service? What is the actual different between them?