Stratigraphic nomenclature

The vast stretch of time that preceded the present day on Earth has now been divided and sub-divided into manageable chunks. These divisions have been made according to changes in the stratigraphy of the rock column; most importantly biostratigraphical changes. Definitions of these time divisions are given below.

Eon  This is the coarsest division of geological time. The Phanerozoic eon, for example, began some 550 million years ago and continues today. This covers the time during which abundant life that readily fossilized has been present on our planet.

Era  The Phanerozoic eon is split up into the Palaeozoic, the Mesozoic and the Ceanozoic eras. These eras are divided by mass extinctions. Between the end of the Palaeozoic era and the start of the Mesozoic some 90% of marine species and 70 % of terrestrial species were wiped out. Between the end of the Mesozoic era and the start of the Caenozoic about 50% of species died out – most notably the dinosaurs and the ammonites.
Eras are sub-divided into periods.

Period  This is the division of geological time most commonly encountered by the amateur palaeontologist. If nothing else, most can confidently state that the rocks of their favorite fossil hunting ground were laid down during the Jurassic, Cretaceous, Silurian, etc. period.

Epoch  An epoch is a division of a period. Many are simply called the Lower/Early, Middle or Upper/Late part or the period they describe, e.g. the Lower Cretaceous, the Upper Triassic, etc. Some, however, have names relating to where the type sections of the epoch are found, e.g. the Ludlow, Wenlock and Llandovery epochs of the Silurian period, which are named after locations in Shropshire, England and Dyfed, Wales.
Interestingly, the Palaeocene, Eocoene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene are not periods as many think but epochs of the Palaeogene (Palaeocene to Oligocene) and Neogene (Miocene to Holocene) periods.  

Stage  The stage sub-division is used by palaeontologists to denote a suite of rock layers within an epoch that all contain a certain easily identifiable and abundant index fossil or fossils. One stage may be represented by numerous different environments around the world however, so stage index fossils are regional in nature.

Zone  Stages can be divided into zones. These are named after fossil species that are abundant, often short lived and relatively restricted to that particular zone.  

Bed  This is a specific layer or stratum of rock that that can be separated from the proceeding and succeeding by some definite change in stratigraphy.