Well, cell/mobile phones are locked by telephone companies so you can’t change your T-Mobile SIM card with a Cingular one for example. They don’t want you to be able to easily change providers.
If you are going overseas, you’ll want to take your phone (it must be Tri-Band), and use a SIM card from a European provider to save loads of money on getting a new phone or for paying long distance fees to Verizon or whoever your American provider is.
Unlocking phones requires entering a code using the keypad on your cell. The code calculator gives you 5 codes.
If the all 5 codes don’t work, your phone will not let you enter in any more codes. So go to any cell phone shop and for about $25-30, they can plug in a cable and unlock the phone.
Unlocking cell phones don’t erase anything and none of the settings are altered.