In Europe where I grew up, we use 900/1800MHz - my original GSM phone wouldn't work in the States when I visited. I had to wait quite a while for the first tri-band phone to come out before I could get one that would work in Canada where I now live where we predominantly use 850/1900. Interestingly 1900 is the predominant band in Canada with 850 being a "backup", but in the U.S. it's determined by regulatory requirements of the location.
A quad-band "world phone" (that supports 850/900/1800/1900) will work in _most_ places (some exceptions) in the world that support GSM. There are some countries that use some obscure bands - Benelux, Russia etc. use 450MHz.
2100 MHz 3G/4G are HSDPA/LTE so your phone needs to support HSDPA/LTE as well as being GSM... and then you also get 2100 MHz LTE on CDMA...
On 2G, 2100MHz was only on CDMA, which most of the world (still) doesn't support... so if you've got a CDMA phone and are travelling anywhere outside of a very limited list of some 45 countries (North/Central/South America, Caribbean, Far East), you're SOL.
So like I said UMTS/HSDPA/HSUPA gets more complicated, because then you're not just talking GSM or CDMA... your phone needs to talk UMTS/HSDPA/HSUPA as well as having the correct frequencies and GSM or CDMA.
