Nothing makes selecting a product more difficult than navigating through datasheets filled with acronyms. Unlike most technophiles, I absolutely hate acronyms. All technologies have their fair share of random letters grouped together that mean something to somebody. But it appears that the engineers in charge of cellular technology got in line twice when the letters where being handed out. And they don't even sound cool... at least you can pronounce ASCII...
The most confusion comes when discussing the type of cellular network. When you ask somebody this question, they will usually answer with the company they send the bill to. But, the answer is actually GSM or CDMA.
GSM stands for Global System for Mobile Communications. It is the most popular standard for cell phone networks in the world. This is the standard that AT&T uses in the U.S. It is also very popular with most international carriers. Besides the technical difference that I will not go into, GSM networks require a SIM card. SIM stands for Subscriber Identity Module. As the name implies, this card identifies your device on the network.
CDMA is short for Code Division Multiple Access. This system is mainly used in the U.S. CDMA systems do not use a SIM card. Instead, your device needs to be programmed. The CDMA network is used by Sprint and Verizon in the U.S.
Now for the G's... GSM and CDMA are both 2G networks. The "G" stands for Generation. The first generation was called AMPS. It was an analog network used way back in the 80's. Second generation GSM and CDMA networks are digital and can handle voice and data - data being what we care about. 2G networks are relatively slow. Third Generation, or 3G networks are faster and can deliver data at speeds comparable to your home DSL router or better.
There are several types of 3G networks.
EV-DO: This stands for Evolution-Data Optimized. It is associated with CDMA networks and can deliver data at speeds ranging from 144 Kbps to 2 Mbps.
UMTS and HSDPA: UMTS stands for Universal Mobile Telecommunications System. HSDPA stands for High-Speed Downlink Packet Access. These are upgrades to GSM networks, and like EV-DO, they provide high speed data throughput. Without getting technical, the only difference between the two is that HSDPA is faster than UMTS and is sometimes called 3.5G. In the U.S., AT&T offers both UMTS and HSDPA.
In between 3G and 2G are the "2.5G" networks. These are generally good enough for most industrial and commercial applications. A variation of the CDMA standard is 1xRTT, or "1 times Radio Transmission Technology." It delivers speeds between 30 Kbps and 90 Kbps. On the GSM side are GPRS and EDGE. GPRS stands for General Packet Radio Service. EDGE is short for Enhanced Data Rates for GSM Evolution. They also deliver between 30 and 90 Kbps data rates