Bishoujo Senshi Sailor Moon (Pretty Soldier Sailor Moon) is a story of a young girl, Usagi Tsukino, living in Tokyo who one day finds a talking cat with a cresent moon on her forhead. The cat, Luna, tells her that she is the pretty soldier of justice, Sailor Moon. She must defend against evil. To do this, she has many friends (Sailor Mercury, Sailor Venus and so forth) that help her. When needed, the girls turn into Sailor Soldiers and use their attacks from the blessings of the planets to defeat the evil. The series currently has five seasons (Salior Moon, Sailor Moon R (split into two different sagas in the anime, both dubbed 'Sailor Moon R'), Sailor Moon S, Sailor Moon Super S and Sailor Moon Stars. In each passing saga, Sailor Moon becomes stronger, but so do the enemies. It is a clasic shoujo manga/anime (girl comic/animation) with lots of 'girl power' and stuff like that.
In Sailor Moon S (S stands for 'Super'), Hotaru Tomo shows up. She eventually becomes Sailor Saturn, the soldier of silence and the goddess of death. She comes from the planet of destruction, Saturn. She is simply a character in the whole Sailor Moon universe, but in Sailor Moon S, she plays a very major role.