Geologic Time Scale

Geologic time is measured in billions of years.  
The earth is about 4.6 billion years old. 
Geologic time is divided into eons, eras, epochs, and periods.  
There are two eons:  Phanerozoic Eon and the Precambrian Time or Eon.  

http://geology.er.usgs.gov/paleo/geotime.shtml

For a more detailed version of the geologic time scale, see the University of California (Berkeley) Museum of Paleontology's excellent Web Geological Time Machine. For a review of the concept of geologic time and how it is measured, see the USGS publication "Geologic Time."

Phanerozoic Eon (544 million years ago to present)

Precambrian Time (4500 to 544 million years ago)

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http://geology.er.usgs.gov/paleo/glossary.shtml#hadean

Hadean

The earliest subdivision of the Precambrian, spanning the time between the formation of the Earth, about 4.5 billion years ago, and the start of the Archaean era, 3.8 billion years ago. This interval predates the period of true geologic time since no rocks of this age are known on Earth, with the exception of a few meteorites

Archaean

The middle era of Precambrian time, spanning the period between 3.8 and 2.5 billion years ago. Life arose on Earth during the early Archaean, as indicated by the appearance of fossil bacteria in rocks thought to be about 3.5 billion years old. Its name means "ancient."

Proterozoic

The final era of the Precambrian, spanning the time between 2.5 billion and 544 million years ago. Fossils of both primitive single celled and more advanced multicellular organisms begin to appear in abundance in rocks from this era. Its name means "early life."

Paleozoic

An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic, spanning the time between 544 and 248 million years ago. The word Paleozoic is from Greek and means "old life."

Mesozoic

An era of geologic time between the Paleozoic and the Cenozoic, spanning the time between 248 and 65 million years ago. The word Mesozoic is from Greek and means "middle life."

Cenozoic

An era of geologic time from the beginning of the Tertiary period (65 million years ago) to the present. Its name is from Greek and means "new life."

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http://pubs.usgs.gov/gip/geotime/divisions.html

Major Divisions of Geologic Time

The major divisions, with brief explanations of each, are shown in the following scale of relative geologic time, which is arranged in chronological order with the oldest division at the bottom, the youngest at the top.

See also:  http://www.ucmp.berkeley.edu/help/timeform.html