UTC

Universal Coordinated Time, or "UTC", is the reference time zone from which all other time zones around the world are calculated. This was formerly known as Greenwich Mean Time and abbreviated as GMT (and is still colloquially called GMT sometimes). The new name was coinged to eliminate having the name of a specific location in an international standard, and because of confusion with English local time (which is the same as UTC during the winter months, but is one hour ahead of UTC in the summer since the introduction of [British Summer Time]?).

UTC's reference point is [Greenwich Observatory]? in Greenwich?, London, England. Theoretically, noon UTC coincides with the sun reaching its highest point in the sky at this location (which is also 0 degrees longitude). Due to the Earth's rotational jitters, UTC slowly gets out of synchronization with this reference and "leap seconds" are added or removed to resynchronize, at either the end of June or December each year.

"UTC" doesn't actually stand for anything; it was chosen as a compromise between the English abbreviation "UCT" and the French abbreviation "TUC".

See http://aa.usno.navy.mil/faq/docs/UT.html, List of countries with their time zone.