[WhiliaStelt]

Then of course there are hybred networks which use cell and CDMA or cell and GSM (one for data and one for calls or for back up range on reception, etc). The technology and combinations can get complicated very quickly so a lot depends on what system you are using.

Having a phone "unlocked" means you can shop around to other carriers. For instance many "world band phones" can work on both GSM and CDMA but not all GSM or CDMA phones use the same frequencies because different countries specify different frequencies. Unlocking the phone let's you use other network carriers on that phone instead of just the one who originally sold you the phone.