Originally, the analogy represented the threat of global nuclear war, but since 2007 it has also reflected climate-changing technologies and "new developments in the life sciences and nanotechnology that could inflict irrevocable harm.
Time changesIn 1947, during the Cold War, the clock was started at seven minutes to midnight and was subsequently advanced or rewound per the state of the world and nuclear war prospects. Setting the clock is relatively arbitrary, and decided by the directors of the Bulletin of the Atomic Scientists reflecting global affairs. The clock has not always been set and reset as quickly as events occur; the closest nuclear war threat, the Cuban Missile Crisis in 1962, reached crisis, climax, and resolution before it could be set to reflect that possible doomsday.
The most recent officially-announced setting — six minutes to midnight — was on 14 January 2010. Reflecting international events dangerous to humankind, the clock hands have been set nineteen times, since its initial start at seven minutes to midnight in 1947.
All this sources and image credits solely goes to http://en.wikipedia.org/wiki/Doomsday_clock