From Wikipedia.org

Sailor Moon (美少女戦士セーラームーン, Bishōjo Senshi Sērā Mūn, officially translated as Pretty Soldier Sailor Moon) is the title of a Japanese media franchise created by Naoko Takeuchi. It is generally credited with popularizing the concept of a sentai (team) of magical girls, as well as the general re-emergence of the magical girl genre itself.
The story of the various metaseries revolves around the reincarnated defenders of a kingdom that once spanned the solar system, and the evil forces that they battle. The major characters—called Sailor Senshi (literally "Sailor Soldiers"; frequently called "Sailor Scouts" in the North American version)—are teenage girls who can transform into heroines named for the moon and planets (Sailor Moon, Sailor Mercury, Sailor Mars, etc). The use of "Sailor" comes from a style of girls' school uniform popular in Japan, the sērā fuku (sailor outfit), after which the Senshi's uniforms are modeled. The elements of fantasy in the series are heavily symbolic and often based on mythology.