Geologic time is a way to place dates on the formation of rocks and organize the information that we have about how the Earth formed, how it has changed, and how living things have changed along with it. In the 1600's scientists began to be interested in rocks and rock formation, but it wasn't until 1815, when William Smith produced the first geologic map, that geology began to become a unified science.
From longest to shortest, the divisions currently in use are eons, eras, periods, epochs, and ages. While the names come from localities where discoveries were made (for example, the Cambrian is for Cambridge, England), the lengths of the various periods and how they are divided has been dictated by the fossils found within them. Whenever the fossils indicate that a major change in the life forms of the planet occurred, scientists place a dividing line between one division and the next. Really big changes, such as the appearance of the first fossils, mark eons. Lesser changes, such as increase in diversity or range of a group of living things, are marked by eras or periods. For example, the appearance of fish in the fossil record marks the change from the Cambrian Period to the Ordovician Period, and the retreat of the glaciers marks the end of the Pleistocene Epoch and the beginning of the Holocene Epoch.