It works fine. You just need to make sure the phone you buy is compatible with European frequencies. Many are, including the iPhone and iPad. Both my AT&T iPhone 4G (unlocked) and iPad 3rd gen work fine on a German sim card. The Verizon iPhone 5 also works.

It used to be the case, but very rarely with new phones.

wrong, as stated below...

technically, baudehlo is mostly correct. Most new phones: androids (HTC, Samsung, etc), iphones and whatever, are able to be used around the world as long as they're unlocked and have SIM card slots because these countries are at least using one or more of these frequencies: 850/900/1800/1900/2100, which most new phones are capable of.

The parts that he's wrong would be those few "dumb" phones that companies still sell that aren't able to use the other frequencies, but this is rare. Also, some countries don't use the same "LTE" or "4g" frequencies that the US uses, so if you were to bring your phone there, it would still work, but it be limited to "3g" instead of "4g" speeds, but this varies country to country and it also depends on the network and plan that you choose.

iPhone 5 from Verizon or AT&T will work with European and Australian GSM, but data will be 3G. LTE does not work in Europe. I am assuming Android phones with similar radios to the iPhone 5 will support the same networks abroad.

that's also because there is nearly no LTE coverage in Europe... besides, who needs that for daily use? 3G is just fine ;)

There is some LTE but on odd frequencies. There is lots of hdspa+ or 3.5g though.

I don't know if this help but I couldn't get a plan at some companies in Canada because my french phone had different frequencies.

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.

It gets a little more complicated when you get to UMTS/HSDPA/HSUPA... but Wikipedia explains it all quite nicely http://en.wikipedia.org/wiki/GSM_frequency_bands

So your iPhone will work everywhere - at least everywhere that uses at least one of the 850/900/1800/1900 MHz frequencies... if it's unlocked.

You forgot 2100MHz 3g.

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.

GSM phones bought in the US work in Brazil, at least.