Carbon-14

Carbon-14 is an isotope of the element carbon, so named because it has an atomic mass of 14. It has an atomic nucleus containing 8 neutrons and 6 protons. 6 is the Atomic number for all carbon isotopes. Carbon-14 is also called “carbon 14” or “radiocarbon”, or in symbols: 14C, or 146C, or C14.
Carbon 14 is the only radioactive isotope of carbon found in nature. It is generated in the Earth’s atmosphere by Cosmic rays and decays with a Half-life of about 5700 years. This long half life makes Carbon 14 exist for enough time to be detected. Other isotopes of carbon are either stable, or have a short half life. 
The amount of carbon 14 that remains after the death of a living thing is a measure of that time. This is useful for as long as 60,000 years. This measurement is radiocarbon dating.
Neutrons in cosmic rays collide with nitrogen in the atmosphere and the collision produces carbon 14 and hydrogen. Plants and other living things take up carbon in the carbon cycle. When a thing dies, there is no source of carbon, and the carbon 14 in the body decays to nitrogen (and also to electrons, antineutrinos, and gamma rays).