Prior to the introduction of standard time, every municipality set their clock, if they had one, but the local position of the sun. This served well until the introduction of the train, when it became possible to travel fast enough to require almost constant re-setting of clocks. After missing a train for just this reason in 1878, Sir Sandford Fleming invented standard time to fix the problem.
Standard time divides the world into 24 "time zones", each one covering, in theory at least, 15 degrees. All clocks within each of these zones would be set to the same time. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard, leading to the widespread use of Greenwich Mean Time in order to set local clocks.
A more detailed article titled Universal Time exists. See also Coordinated Universal Time.
wikipedia.org dumped 2003-03-17 with terodump