An atomic clock is a highly precise timekeeping device which relies on the known frequency standard of an element to mark the passing of time. Every element emits pulses at known intervals in different frequencies, and these emissions are highly stable and very regular. With the assistance of these pulses, it is possible to keep time very accurately. Time can be marked so accurately, in fact, that the international standard for the definition of a second is based on cesium 133, an element used in the most precise atomic clocks in the world.
Atomic clocks rely on the use of an oscillator that is tuned to the frequency in which emissions occur. In a highly controlled environment designed to eliminate as much uncertainty as possible, the oscillator responds to the pulses generated by the element used in the atomic clock. Although many people associate "atomic" with nuclear radiation, an atomic clock does not actually use elements which generate dangerous radiation.
The first atomic clock was developed in 1948. It was not terribly accurate, but it demonstrated that the theories developed in earlier decades were in fact true, and it was possible to build a clock which used the frequency standard of an element as a timekeeping measure. Researchers quickly developed more accurate clocks, and eventually a series of cesium fountain clocks were in use around the world, keeping Universal Standard Time, the basis which is used for determining what time it is. These clocks are used by national governments and scientific labs to ensure that their timekeeping is accurate and uniform.
Laboratories use atomic clocks to keep highly accurate time. A number of companies also sell clocks which are labeled as "atomic" to consumers. These clocks are actually radio clocks; they communicate with other atomic clocks to keep time, rather than being atomic clocks themselves. While these clocks can be very accurate, there is a margin for error which makes them unsuitable for scientific use.
In the United States, the National Institute of Standards and Technology (NIST) maintains a cesium fountain atomic clock in a lab located in Boulder, Colorado, which is used as one of numerous clocks which contribute to the coordinated timekeeping used to keep track of International Atomic Time. Other participating labs can be found in locations like Britain, Australia, Egypt, India, Russia, and France, among many others. The accuracy of these clocks is constantly improving; as of 2009, such clocks should not gain or lose a second in a 60 million year period.