Terminology

The major divisions are shown in the scale of relative geologic time (Table 1), which is arranged in chronological order with the oldest division at the bottom, the youngest at the top.

Eons are the largest intervals of geologic time and are hundreds of millions of years in duration. In the time scale the Phanerozoic Eon is the most recent eon and began 570 million years ago. Eons are divided into smaller time intervals known as eras. In the time scale the Phanerozoic is divided into three eras: Cenozoic, Mesozoic and Paleozoic. Very significant events in Earth's history are used to determine the boundaries of the eras.

Eras are subdivided into periods. The events that bound the periods are wide-spread in their extent but are not as significant as those which bound the eras. In the time scale above you can see that the Paleozoic is subdivided into the Permian, Carboniferous, Devonian, Silurian, Ordovician and Cambrian periods.

Finer subdivisions of time are possible and the periods are subdivided into epochs. Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units – e.g. "Upper Jurassic", "Middle Cambrian". Upper, Middle, and Lower are terms applied to the rocks themselves, as in "Upper Jurassic sandstone," while Late, Middle, and Early are applied to time, as in "Early Jurassic deposition" or "fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. It is a key aspect of the work of the International Commission on Stratigraphy to reconcile this conflicting terminology and define universal horizons that can be used around the world.

(http://en.wikipedia.org/wiki/Geologic_time_scale)

Geologic Time Scale

EON	ERA	PERIOD	EPOCH	AGE (my)
PHANEROZOIC	CENOZOIC	Quaternary	Holocene	0.01
			Pleistocene	1.6
		Neogene	Pliocene	5.3
			Miocene	24
		Paleogene	Oligocene	37
			Eocene	58
			Paleocene	66
	MESOZOIC	Cretaceous	Late	144
			Middle
			Early
		Jurassic	Late	208
			Middle
			Early
		Triassic	Late	245
			Middle
			Early
	PALEOZOIC	Permian	Late	286
			Middle
			Early
		Carboniferous	Late	360
			Middle
			Early
		Devonian	Late	408
			Middle
			Early
		Silurian	Late	438
			Middle
			Early
		Ordovician	Late	505
			Middle
			Early
		Cambrian	Late	570
			Middle
			Early
PROTEROZOIC	Also known as Precambrian			2500
ARCHEAN				3960