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Relative dating observes the placement of fossils and rock in layers known as strata. Basically, fossils and rock found in lower strata are older than those found in higher strata because lower objects must have been deposited first, while higher objects were deposited last. Relative dating helps determine what came first and what followed, but doesn't help determine actual age.
Radiometric dating, or numeric dating, determines an actual or approximate age of an object by studying the rate of decay of radioactive isotopes, such as uranium, potassium, rubidium and carbon-14 within that object. Radioactive isotopes decay at a fixed rate. This rate provides scientists with an accurate measurement system to determine age. For example, carbon dating is used to determine the age of organic materials. Once something dies, it ceases taking in new carbon-14, and the existing carbon-14 within the organism decays into nitrogen at a fixed rate. Scientists measure the proportion of carbon-14 left in the organism to determine its age.
