This was the first method developed to date very old rocks. U-238's half-life is 4.468 billion years (the standard age of the earth). U-235 has a half-life of 704 million years, and Th-232 14 billion years. Each decays, in steps, to other elements including polonium, and finally lead (Pb).
The different "parent" isotopes yield different "daughter" isotopes of lead: U-238 yields Pb-206; U-235 yields Pb-207; Th-233 yields Pb-208. Abundances of these lead isotopes in a sample are measured and compared with the abundances of the remaining parent isotopes to determine the sample's age.
2010 Arthur V. Chadwick, Ph.D.