GSM now stands for Global System for Mobile Communications. Originally, when it was only some E.U.-regional standard it stood for "Groupe Speciale Mobile" ("Special Mobile Group" in French).
A standard of cellular telephony of second generation - one of 2G standards. It was invented in E.U. and applied at first there as local standard (countries were forced by governments to use it), but it grew to current situation, when it is considered global standard.
GSM1900 is used in USA. GSM1800/GSM900 are used in E.U., parts of Asia (except for Korea and Japan) and in some other places.
