[Bossman]

A phone needs to be unlocked so that you can put any carrier's sim card in it. Subsidized phones are usually locked to a carrier and will only take that carrier's sim card. So, say you buy a phone on AT&T or tmobile's website they will only take an AT&T and tmobile sim respectively. So, if you want to use thet phoen with say a prepaid sim you buy in europe or if you want to put a tmobiel sim in the AT&T phone or vice-versa, you will need to get it unlcked. This can be done for free by contacting your carrier or by doing it via a 3rd party. Tmobile and AT&T will give the unlock code for their phones as long as you have been a customer in good standing for at least 90 days. OTOH, you can just buy an unlocked phone from ebay, amazon, buy.com, etc. and you are all set.