In 1815, William Smith, a surveyor, canal builder, and amateur geologist from England, produced a geologic map of England and in doing so demonstrated the validity of the principle of faunal succession. This principle simply stated that fossils are found in rocks in a distinctive order. This principle led others to use fossils to define increments within a relative time scale.
The history of the earth is divided into a hierarchical set of age groups. As increasingly shorter units of time, the generally accepted divisions are eon, era, period, epoch, and age. These time periods reflect the history found in the geological record.
In more recent times, geologists have attempted to place absolute dates on the boundaries between the time periods. There are a variety of reasons for doing so, but they include better insight into biological evolution, adaptive radiations, extinctions and recoveries, climate change, and catastrophes.
There are diagrams for three timescales on this page. The first shows how absolute ages for part of the timescale have varied through time. The second is the timescale of the International Commission on Stratigraphy; and the third is the timescale of the Geological Society of America, developed for their successful Decade of North American Geology project.
Nick Christe-Blick has some interesting thoughts on Geological Time Conventions and Symbols.
More detailed timescales: Devonian timescale (pdf): A paper (1013 kb .pdf) describing the methodology for the revised calibration is available here.