What is an Atomic Clock? Explain it in detail. Mention all the key terms

An atomic clock is the most accurate type of timepiece in the world, designed to measure time according to vibrations within atoms. NIST-F1, the United States' standard atomic clock, is said to be so accurate that it would neither gain nor lose a second in over 30 million years. Atomic clocks are used to coordinate systems that require extreme precision, such as Global Positioning System (GPS) navigation and the Internet. A group of atomic clocks located in a number of places throughout the world is used in conjunction to establish Coordinated Universal Time (UTC).