There are various types of cellular phone standards available, mainly GSM, CDMA, and TDMA. GSM is by far the widest used and many countries that used to embrace especially TDMA are sitching to GSM. You may, however, wonder what each of these services are and what they mean.

GSM - The Global System for Mobile Communications is the most popular standard for mobile phones in the world. It is used by over 2 billion people across more than 212 countries and territories. The GSM standard makes international roaming possible between mobile phone operators, al subscribers to use their phones in many parts of the world. GSM signaling and speech channels are Digital call quality, which means that it is considered a second generation mobile phone system.

CDMA - Code Division Multiple Access is a form of multiplexing and a method of multiple access that does not divide up the channel by time or frequenc. What it does is encode data with a special code that is associated with each channel and uses the constructive interference properties of the special codes to perform the multiplexing. In addition, CDMA also refers to digital cellular telephony systems that make use of this multiple access scheme, such as those pioneered by Qualcomm, and W-CDMA by the International Telecommunication Union or ITU.

TDMA - Time Division Multiple Access is a channel access method for shared medium networks. It allows many users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using his own timeslot. This allows multiple stations to share the same transmission medium, like radio frequency channel, while using only the part of its bandwidth they require. TDMA is used in the digital 2G cellular systems such as GSM and IS-136 and cordless phones. It is also used extensively in satellite systems and combat-net radio systems.

More information can be found on Wikipedia.