"Bishoujo Senshi Sailor Moon" (Japanese for "Pretty Soldier Sailor Moon") or simply "Sailor Moon" for short, is a very popular animé/manga created by Naoko Takeuchi, and used to be the basis for this chat (thus the name Fake MOON Chat, the Sailor Moon Chat room, The Negaverse room, the "SM" logo on nearly every page of the site, and the over-abundance of icons), which was originally created by Sailor Sam, and simply called Moon Chat.
Basically, it's the story of a ditzy teenage girl named Usagi Tsukino, who is a student of Crossroads Junior High School and finds out that she is the guardian/long-lost princess of an ancient civilization on the (Earth's) Moon, the ruling kingdom of the galaxy. The story takes place in Tokyo, Japan, where strange things keep happening and a mysterious black cat named Luna, who later becomes Usagi's pet cat, as well as her guardian/advisor/friend, is the one who gives her the magical brooch she uses to transform into Sailor (because of the sailor-suited uniforms they wear in Japan) Moon, "the champion of love and justice". Later, she is given/acquires other items which allow her to transform into more powerful versions of her sailor-suited self, Super Sailor Moon, and Eternal Sailor Moon.
There are other Sailor Senshi (Japanese for "warrior(s)", although we call them Sailor Scouts here in Canada/America) who appear one-by-one, with the exception of Sailor Neptune and Uranus, who show up together. There is a Sailor Senshi for each planet, (who are also long-long princesses of their respective planet) with the exception of Earth, who is represented by Tuxedo Mask, the prince of Earth (son of Gaeia). Unlike Sailor Moon, the other Sailor Senshi use wands, not brooches to transform, (Tuxedo Mask uses a rose) but like Sailor Moon, they also have a "super" form, which they can later transform into. Sailor Moon, being the main character, is the only one who has an "eternal" form, while Tuxedo Mask's form never changes at all.
Artemis (who is actually a male, not a female, despite the name) is the other (white) cat guardian/advisor to the girls and shows up at the same time as Sailor Venus, A.K.A. her popular Hollywood personality, Sailor V, since he's her cat. Actually, Naoko Takeuchi created a "Sailor V" manga before she created "BBSM", thus the "Sailor V" Manga Characters category.
There are three teams of Sailor Senshi; the "inner" Sailor Senshi, (consisting of Sailor Moon, Sailor Mercury, Sailor Mars, Sailor Jupiter, and Sailor Venus) and the slightly more powerful "outer" Sailor Senshi (consisting of Sailor Neptune, Sailor Uranus, Sailor Pluto, and the most powerful Sailor Senshi, Sailor Saturn), and the Sailor Starlights (consisting of Sailor Star Maker, Sailor Star Healer, and the team's leader, Sailor Star Fighter. They serve Princess Fireball and are strangely enough men in their human forms, but transform into women Sailor Senshi).
Together, the Sailor Senshi encounter all kinds of monsters and there is a main boss/leader for every sub-series, the first of which being Queen Beryl, and the last one being Galaxia. Enemies will not usually have their own category, but can be found in the name of their group/team.
Spoilers (which are mentioned to explain the presence of icons of these characters but will not be discussed to any great detail): There is another Sailor Scout named Sailor Chibi-Moon (or Chibi-Usa in her human form) and she has her own (gray) cat, Diana. There is also another Sailor Senshi named Sailor Chibi-Chibi Moon (or Chibi-Chibi in her "human" form).
"BSSM" related emoticons: