Coordinated Universal Time.

UTC is french - Temps Universel Coordonné. In translates to Universal Coordinated Time. It is essentially the same as Greenwich Mean Time. The military calls it Zulu.

Universal Time Coordinated.

UTC is a compromise between CUT (for the English Coordinated Universal Time) and TUC (for the French Temps Universel Coordonné).

Both are time zones. GMT is Greenwich Mean Time, the official time at the Royal Observatory in Greenwich, England. UTC is Universal Time Coodine', or in English "Coordinated Universal Time." For practical purposes it is pretty much the same thing, but it's more acceptable around the world because it doesn't sound quite so British.