Coordinated Universal Time, or UTC, is the reference time zone from which all other time zones around the world are calculated. It is the successor of Greenwich Mean Time, abbreviated as GMT, and is still colloquially called GMT sometimes. The new name was coined to eliminate having the name of a specific location in an international standard.
Unlike GMT, UTC is not kept by watching the Sun or stars, but is measured by atomic clocks. Because the rotation of the Earth slows down, it lags behind this atomic time. In order to keep the clock time UTC synchronized to the day and night of UT1, leap seconds are added (or removed) at either the end of June or December whenever necessary. This is announced by the International Earth Rotation Service at http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.dat .
"UTC" is not a real abbreviation; it is a variant of universal time, abbreviated UT, and has a modifier C (for "coordinated") appended to it just like other variants of UT. It may be regarded as a compromise between the English abbreviation "CUT" and the French abbreviation "TUC".