Unlocked GSM phone means that it wouldn't be locked into a specific carrier network. On the upside it means freedom to choose your own GSM carrier that suits your needs (and wallet) best. On the down side several carrier-specific features may not be available.
If you lived in US make sure that the unlocked phone is compatible with US GSM frequencies. International phones or EU-spec phones may have problems connecting to US carrier network due to different regulation regarding comm. frequencies.
Unlocked cell phone means You can use with different network. Normally all mobiles locked by particular provider. So you cannot switch over from another network. In this situation you must unlock your cell phone.
There are lot of advantages available to unlock your gsmphone:
1)Once unlocked your mobile you can use it with different network
2)You can resale your mobile at higher price
3)Roaming charges is also cut.
If you need blackberry unlock code you can visit this site UnBBLock.com here you can get the code and unlock you mobile.