Here's a breakdown of geologic time divisions from longest to shortest:

* Eon: The largest division of geologic time, representing hundreds of millions to billions of years. Examples include the Precambrian Eon and the Phanerozoic Eon.

* Era: A subdivision of an eon, typically lasting tens to hundreds of millions of years. Examples include the Paleozoic Era, Mesozoic Era, and Cenozoic Era.

* Period: A subdivision of an era, typically lasting tens of millions of years. Examples include the Permian Period, Jurassic Period, and Quaternary Period.

* Epoch: A subdivision of a period, typically lasting millions of years. Examples include the Pleistocene Epoch, Holocene Epoch, and Eocene Epoch.

* Age: The smallest formal division of geologic time, representing a shorter time span within an epoch.

So, while you might be familiar with periods like the Jurassic or the Cretaceous, the Eon encompasses the largest chunk of Earth's history.