GSM, or Global System for Mobile Communications, is a cellular phone protocol that is popular in many parts of the world, and is a chief competitor to the CDMA protocol in the United States. In fact, for those who have CDMA phones, they may find if they go to other parts of the world, such as Europe, their cellular phones will not work. The GSM mobile phone standard is more popular in more places, simply because the standard has been around longer, and was considered by many to be superior to CDMA, at least initially.

In the United States, GSM operates on frequencies in the range of 1.9 GHz and 850 MHz. The 850 MHz range is also used in many other countries, such as Australia and some in South America. In Europe, GSM operates in the ranges of 900 MHz and 1.8 GHZ. The ranges are not that important to consumers, who are generally more concerned about transmission speeds and reliability.

The GSM system is capable of data transmission speeds of up to 9.5 kilobits per second. This used to be a feature that other standards could not compete with. Since then, however, other technologies such as CDMA have improved, putting most cell phone standards on a similar playing field. Therefore, those who are comparing the different cell phone standards may not be as concerned about data transfer speeds as they once would have been.

One of the distinctive features of the GSM system is the use of SIM cards. This card will store all personal data and contacts on it. Once a phone is traded for a new one, transferring the data and activating the phone becomes simply a matter of changing the card. The contact list, calendar, and any other personal data will seamlessly transfer to the new phone. Accessing it may be slightly different, depending on the phone's software, but the information will be the same. Only GSM offers this, though there are plans to eventually offer a similar service for CDMA customers in the United States.

Consumers in some areas may only have access to GSM or CDMA, but not both. This is becoming increasingly rare, however, and consumers have a choice, more often than not, based on which provider they are using. Most cellular service providers only offer service in one standard or the other. Supporting two standards would be redundant and costly.