Each Era can be won in any of four ways. These are called the victory conditions, and they are based on the four major avenues of achievement -- gold, culture, science, and military conquest. The current Era victory conditions are visible by hovering over the Era icon at the top right of the City screen, or by visiting the World screen. Note that only Civilizations can win Eras -- you must be a member of one to accomplish a win! View the Civ World wiki page on Eras for more information.