CDMA vs. GSM
Not all cellular devices can be used on all networks, as US carriers don’t all run on the same kind of technology.
Cellular companies operate under one of two types of networks; CDMA or GSM.
Out of the biggest US carriers, AT&T and T-Mobile run on GSM networks, while Sprint and Verizon Wireless use CDMA.
You can find a full list of US GSM carriers here.
GSM-compatible devices use a SIM card, a small removable plastic card that stores the data needed to identify a subscriber on a wireless network.
You can remove your SIM from your current device and insert it into another compatible device, provided it’s either with the same carrier or you’ve ‘unlocked’ it from its original network.
In contrast, while phones running on CDMA networks can be unlocked from a network, they will usually need to be reprogrammed to work perfectly with a different carrier, as CDMA networks use electronic serial numbers to identify subscribers, and don't have a removable SIM card.
You can find more information regarding the difference between GSM and CDMA here.