Before modern radiometric dating techniques made it is possible to determine rock ages empirically the geologic record was divided into relative time units based on correlation of the rock formations, mostly by the index fossils they contained. This was the basis for the modern Geologic Time Scale-- a concoction of names like Cretaceous, Jurassic, and Ordovician, assigned by early European geologists.
Today, scientists have tools available to them to determine the absolute age of certain rocks. This is accomplished by measuring the amounts of certain radioactive isotopes contained in the rocks (usually limited to igneous rocks), in relation to the amount of corresponding decay isotopes (daughter products) that are present. By determining absolute age dates of hundreds of key rock beds around the world, together with a little detective work, it has been possible to assign reasonably accurate absolute age dates to the world-wide Geologic Time Scale.