The radioactive system used to date things older then about 40,000 years ago is the Potassium-argon dating process. otherwise things younger then that use carbon 14 dating. You can read about dating methods here: http://en.wikipedia.org/wiki/Absolute_dating
Radiometric age dating is used to determine the absolute age of a fossil. It also measures the profusion of radioactive element in a fossil as well as an associated rock. Read more on www.encyclopedia.com.