Time intervals are given names from various regions of the world where rocks of that time are best developed. The usage of these names has evolved over several hundred years and are still changing on the basis of new information. Ages are based on radiometric determinations and are also constantly being refined. Numerous unconformities appear within these geologic units and cannot be shown at this scale. The first term is a time designation and the second refers to the rock deposited during that time interval.
Proterozoic is an eon which is a higher category than era.
Other names, however, are used in different parts of the United States and internationally. The age, in millions of years, is the total time range for a particular geologic period, except for the Tertiary and the Quaternary Periods, where the age is only for the named epoch.