An atomic clock is an exact clock that works with atoms, as opposed to most other clocks which are mechanical. The frequency comes from the crossing radiation of electrons. Atomic clocks are currently the most exact clocks of the world. They are also called primary clocks.
World wide, there are over 260 atomic clocks at over 60 different places. All data is collected at the International Bureau of Weights and Measures in Paris, France. The International Atomic Time is calculated there.