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The geological time scale is a system of chronological measurement that relates rock strata to time. It's used by geologists and other scientists to describe the timings and relationships between events that have occurred during the history of Earth. At the top of this scale is the eon, which is the largest time unit and spans hundreds to thousands of millions of years. Earth's history is divided into four eons: the Hadean, Archean, Proterozoic, and Phanerozoic.

Eons are further divided into eras, periods, and epochs. The vastness of an eon can be hard to comprehend, but to put it in perspective, the Phanerozoic Eon—which we are currently in—started about 541 million years ago with the emergence of abundant complex life.

After eons, the next largest division of the geological time scale is the era. Eras are typically hundreds of millions of years in duration and are marked by significant events such as major extinction events and the diversification of life. For example, the Phanerozoic Eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.

The Paleozoic is known for the development of early life forms, the Mesozoic is famous as the age of reptiles (dinosaurs), and the Cenozoic, which is the current era, is referred to as the age of mammals. These eras give scientists and students alike a way to compartmentalize Earth's history into segments that are characterized by distinct biotic evolutions.

Zooming in on the geological time scale, we come to periods, which are smaller than eras but still span tens of millions of years. Each era is divided into several periods. As an example, the Mesozoic Era includes the Triassic, Jurassic, and Cretaceous periods. These divisions often reflect major shifts in climate and tectonic activity, which in turn influence life on Earth.

Periods can be identified by a distinct set of fossil fauna and flora, and changes in these life forms are indicative of transitions from one period to another. The end of the Cretaceous period, for instance, is marked by the mass extinction that led to the demise of the dinosaurs and opened the way for mammals to become dominant.

The smallest unit in the trio of era, period, and epoch is the epoch. It spans millions of years, and periods are subdivided into these shorter epochs. For example, the current period, the Quaternary, is divided into two epochs: the Pleistocene and the Holocene. The Pleistocene is characterized by the last ice ages, while the Holocene represents the last 11,700 years after the last major ice age, during which all human civilization developed.

Epochs allow for a more granular view of Earth's history and are useful for studying the changes that have occurred on a more detailed scale, particularly in terms of climate change, evolution, and geological events that affect the distribution of species and the structure of ecosystems.