It helps to understand the alphabet soup of standards and protocols that constitute the global communications network.
GSM and CDMA are the dominant voice technologies in digital wireless. But there is a huge difference between their market shares inside and outside of the United States. Globally, GSM is by far the leader, with about 3 billion users in some 212 countries or territories, accounting for 82% of all subscribers, according to industry statistics. There are four common frequency bands used for GSM, so tri- or quad-band capable handsets are needed for global roaming.
A GSM phone uses a SIM card, which contains a user's subscription information, number and phonebook. SIMs and phones are interchangeable -- except that operator-subsidized phones common in the U.S. are often locked to accept only that carrier's SIMs (some carriers will unlock your handset for travel on request).
GPRS, EDGE, UMTS, HSDPA and EV-DO are key wireless data protocols of varying speeds, available according to the condition of the provider's network.
IMAP/POP3/SMTP are messaging protocols commonly used to access email over the Web from servers. Many handsets include clients to use these protocols. Newer handsets also use Synchronization Markup Language (SyncML), a platform-independent standard for exchanging information (contacts, calendar items, email, etc) between a mobile device and a server or Web service.