UTC is the official term for what is known under Greenwich Mean Time (yes joe, I've learnt, it's mean not main ) You can also call it zulu-time or simply worldtime. UTC has been used as time code for the civil aviation for some years. Before that they mostly used zulu, wich is just the military version of UTC.
12:00 UTC = 1200z (spoken: one thousend two hundred zulu [/ z-time]) = 12:00 GMT = 1300a (spoken: one thousend tree hundred alpha) = 13:00 British Summer Time