This article is from the Anime FAQ, by Steve Pearl with numerous contributions by others.
Sailor Moon is a Bandai television series initially aimed at 14 year-old girls. The main characters are Tsukino Usagi, Mizuno Ami and Hino Rei (true through episode 21). Usagi is a ditzy blonde with looong ponytails and was the first of the girls to become a "se-ra- fuku bishojo senshi", or sailor-suited cute girl warrior (at this point the character designs/costumes began picking up male fans). Ami and Rei joined the team in episodes 8 and 10, respectively. Kino Makoto joined the group at episode 25.
In episode one Usagi is given a brooch that can transform her into a superhero dedicated to fighting the forces of the "Dark Kingdom". Her costume is an abbreviated, and I *do* mean abbreviated, version of her school uniform. Her superhero name is Sailor Moon. Ami becomes Sailor Mercury, Rei becomes Sailor Mars and Makoto becomes as Sailor Jupiter.
The girls gain special attack powers that derive from the kanji that spell the planet their name represents (their real names are spelled with the same kanji, by no coincidence). Thus, Tsuki-no Usagi is Sailor Moon and throws her crescent-shaped tiara (which glows and acts like a killer frisbee). Sailor Mercury (Mizu-no Ami) can hurl bubbles of fog to confuse and disorient. Sailor Mars (Hi-no Rei) throws fireballs. Sailor Jupiter (Ki-no Makoto) Lightning. She calls it "Supreme Thunder". She uses a deadly whirwind of roses in the manga.
The series is campy and parodies many different series. One episode even has the team visiting an animation studio. I think most of its appeal is in its humor and its cult-like following. Great anime it isn't. Great fun it is. (From a posting by Chris Swett with corrections by Robert DeLoura)
Addendum: Sailor Moon (aka Serena) has made her American debut on American TV. Please refer to Sailor Moon FAQ or the Sailor Moon newsgroup for more information. (See the Resources section of the FAQ).