Global System for Mobile communications (GSM: originally from Groupe Spécial Mobile) is the most popular standard for cellular networks for mobile phones in the world.

See  http://en.wikipedia.org/wiki/GSM for more information.

More details about GSM:

 http://students.ee.sun.ac.za/~gshmaritz/gsmfordummies/intro.shtml

 http://www.informatik.hu-berlin.de/~goeller/isdn/GSMDmChannels.pdf

The standards:  http://openbsc.osmocom.org/trac/wiki/GSMStandards

Books:

"GSM Networks: Protocols, Terminology, and Implementation" by Gunnar Heine is particularly useful for starting out.