In common usage, an Era is a (usually lengthy) period of time with common characteristics. For example -- the Biblical era, the Roman era. In Geology, Era refers to four well defined time spans covering the history of the Earth. From oldest to youngest, these are the Proterozoic, Paleozoic, Mesozoic, and Cenozoic eras. In Geology, eras are grouped into still longer periods called Eons.
ERA is also an acronym for Equal Rights Amendment -- an amendment to the United States Constitution that would have guaranteed full equal rights for women in the United States. The ERA narrowly missed adoption in 1982 when it fell 3 states short of adoption. See http://www.equalrightsamendment.org/era.htm.