GSM stands for Global System for Mobile Communications, and it is the network used by the four major cell phone carriers in the United States. GSM-900 and GSM-1800 are used primarily throughout Europe, Australia, Africa, Asia and the Middle East.
Many GSM devices use a SIM card, which allows users to switch devices easily. SIM cards help phones connect quickly to the network of choice and allow users to move contacts from one phone to another.
GSM uses signals that travel from a mobile station to a base station. In the United States, the most common GSM used is 850 MHz and 1,900 MHz.