The Absolute Geologic Time Scale
By modern scientific calculations, the earth is many millions of years old.
The Absolute Geologic Time Scale has been developed by using a process called
"Isotopic Dating", in which the decay rates of certain radioactive materials are
established and measured, then used as "clocks" to calculate the ages of various
rocks. 
The chart below shows how old certain rocks are in millions of years. For example, look at the "Pliocene". Under "Dates" you will see that the Pliocene was deposited 2 to 5 million years ago. The Cambrian was approximately 505-570 million years ago.
The PG is not as interested in the age of rocks in years as she is in the relative sequence of their deposition, and the time period to which they belong. That is because the PG knows that certain conditions existed on earth during each of the geologic time periods. Knowing that a certain rock was deposited in the "Pennsylvanian", for example, may help the PG to interpret rocks in a certain area.
Also, petroleum geologists are mainly interested in rocks from the Mesozoic and Paleozoic Eras. This is because almost all of the oil and gas found so far is contained within these rocks...60 to 600 million years old, approximately. These rocks represent only a small fraction of the total age of the earth, which is measured in billions of years.
The Geologic Time Scale is illustrated here to help you see the way geologists have broken down geologic time into sections, each with an interesting name, and each containing a package of rocks deposited during a certain time period....all over the world.
Time Scale taken from: MODERN PHYSICAL GEOLOGY, Graham R. Thompson Ph.D., Jonathan Turk Ph.D., Saunders College
Words on this chart: neogene, paleogene, cretaceous, jurassic triassic permian carboniferous pennsylvanian mississippian devonian silurian ordovician cambrian ecocambrian cenozoic mesozoic paleozoic prepaleozoic precambrian algae bacteria protoplanet meteorite