How geologists designate the different time periods in earths ancient past?

Geologists divide Earth's history into supereons (there's only one, the precambrian), supereons are divided into eons (there are four), eons are divided into eras, eras are divided into periods (Periods may be the most common divisions you read about. The Jurassic is a period), periods are divided into epochs, epochs are divided into ages.

After the first era (the Cryptic) The various divisions are mostly determined buy the emergence of some form of life.