There is 3 different carrier unlocked devices.
1. GSM Unlocked:  Means that you can be able to use T-Mobile or AT&T Network. 
2. CDMA Unlocked:  Means that you can be able to use Verizon or Sprint Network. But, Sprint Network customer has to check compatibility with your own carrier.

3. Global Unlocked:  Means that you can use all networks. But, Sprint Network customer has to check compatibility with your own carrier.