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A geologic 'era' is a subdivision of geologic time that divides an Eon into smaller buckets. The Phanerozoic Eon is divided into three such timeframes: the Paleozoic, Mesozoic, and Cenozoic represent the major stages in the macroscopic fossil record. These eras are separated by catastrophic extinction boundaries, the P-T boundary between the Paleozoic and the Mesozoic and the K-T boundary between the Mesozoic and the Cenozoic. There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.
The Hadean, Archean and Proterozoic eons were as a whole formerly called the Precambrian Era. This covered the four billion years of Earth history prior to the appearance of hard-shelled animals. More recently, however, those eons have been subdivided into eras of their own.

Era	Timeframe (Ma = million years ago)
Cenozoic	66 Ma to present day
Mesozoic	252 Ma–66 Ma
Paleozoic	542 Ma–252 Ma
Neoproterozoic	1000 Ma–542 Ma
Mesoproterozoic	1600 Ma–1000 Ma
Paleoproterozoic	2500 Ma–1600 Ma
Neoarchean	2800 Ma–2500 Ma
Mesoarchean	3200 Ma–2800 Ma
Paleoarchean	3600 Ma–3200 Ma
Eoarchean	3800 Ma (unofficial)–3600 Ma
Hadean Eon ''not officially divided into eras''	Formation of Earth to 3800 Ma