An era is a period of time in Civilization Revolution. There are four of them in the game: the Ancient Era, the Medieval Era, the Industrial Era, and the Modern Era. Each era has its respective units, buildings, and technologies. Your cities and buildings may also change in appearance when you advance to the next era.
A new era is unlocked once a civilization has researched sufficient technologies. A civ must acquire 5 techs to advance to the Medieval Era, 14 techs to advance to the Industrial Era, and 24 techs to advance to the Modern Era.
Each civilization has a different bonus for each era, which activates when that era is reached.