GlobalSystemForMobileCommunication is better known as GSM. You probably have a MobilePhone based on this. The alternative of CDMA appear to be still very much restricted to Bell related ventures.
In theory a "roaming service" allows you to travel all over the world with a single MobilePhone. In practice this is only recently feasible (if you exclude Japan and South Korea), due to variances in frequencies used.
See http://www.gsmworld.com/roaming/gsminfo/index.shtml for roaming and coverage (include satellite).
see also CodeDivisionMultipleAccess (CDMA)