Radiocarbon dating is a method of estimating the age of organic material.It was developed right after World War II by Willard F.
Some examples of the types of material that radiocarbon can determine the ages of are wood, charcoal, marine and freshwater shell, bone and antler, and peat and organic-bearing sediments.
Age determinations can also be obtained from carbonate deposits such as calcite, dissolved carbon dioxide, and carbonates in ocean, lake, and groundwater sources.
Cosmic rays enter the earth's atmosphere in large numbers every day and when one collides with an atom in the atmosphere, it can create a secondary cosmic ray in the form of an energetic neutron.
(You can read up on radioactivity and isotopes here).
Carbon-14, the radioactive version of carbon, is rare — it only makes up one trillionth of all the carbon in the world.