Sean
SIM card stands for Subscriber Identification Module. It is a small memory card used in GSM phones that contain unique information that identifies the owner of the phone to the mobile network used by his carrier. The SIM card is provided by the GSM wireless carrier and is needed to be able to make calls, send texts, or use mobile internet data through the carrier’s GSM network.
GSM stands for Global System for Mobiles and is the wireless network technology used in the US by AT&T and T-Mobile, and used exclusively by every country outside of the US except for South Korea.
SIM cards on GSM phones are transferrable, which means you can transfer the card to a different GSM phone and your phone number and carrier plan details will transfer with it to the new phone. You can then immediately start making calls and sending texts from the new phone without getting approval through the carrier.
With a GSM phone, you can travel to a different country and use your same phone just by swapping in a SIM card from that country. You will then be able to use talk, text, and data from a wireless carrier in that country.
Note that in the US, Verizon and Sprint use the CDMA network rather than the GSM network. CDMA stands for Code Division Multiple Access and is an older wireless technology that is only used within the US and only by Verizon and Sprint.
Originally, cellphones that used CDMA did not use a removable SIM card. Instead, the device itself contained the identifying numbers and information. This meant that CDMA phones could not easily be switched from one carrier network to another.
Most newer phones however work with both GSM and CDMA technologies, so you don’t have to worry if you bought the phone with one carrier and want to switch to another. An older phone on the other hand might be restricted to either GSM or CDMA, depending on the wireless carrier you bought it from.
