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)