Bishoujo Senshi Sailormoon (or "Pretty Soldier Sailormoon") is a magical girl show that features 
the adventures of fantastical girls in a dramatic world. It was first published in "Nakayoshi," a serial magazine for young girls, as a manga (comic) from February 1992 to March 1997. The anime show based off the manga ran simultaneously and both influenced each other to continue and create new episodes. (For example, the anime created the Doom Tree series so that the manga could make the Black Moon arc and catch-up; executives of the anime urged the creation of the Infinity Arc after the success and public demand for more Sailormoon anime.) The public praises BSSM as the first manga to combine the magical series with the action genre typically associated with boy-related manga.
The creator of BSSM is the artist Takeuchi Naoko, who illustrated all the manga pictures and developed the storylines of the senshi. She released it all in an 18 volume series and also published six artbooks of her colored manga. You can see her page here.
BSSM is not an original story in itself. It is based off Naoko's first "Sailor-suited heroine" SailorV. SailorV's comic, Codename wa SailorV (The Codename is SailorV), appeared in the "Run-Run" magazine as a sister to "Nakayoshi." Naoko was given the chance to draw a story of anything she liked; so, she chose a superheroine who would fight for justice. She based her heroine's costumes on the typical girls' school wear, the seeraa fuku (sailor suit), to get the attention of the young girls who wear them. The standard female objects of normal life are turned into weapons of great power and importance in Naoko's magical girl mangas.
Codename wa SailorV features the adventures of Aino Minako and her magical cat Artemis. With his help, she tranforms into the powerful 
warrior SailorV and fights against all kinds of evils. Bishoujo Senshi Sailormoon follows the story of Tsukino Usagi and her magical cat who find other Sailor Soldiers to help fight against the evils that attack the earth. The SailorV manga eventually ends where Minako joins up the Sailor Team and becomes Sailorvenus in the BSSM manga.
There are many parallels between Codename and the Sailormoon manga. For instance, the "Dark Agency" in the former relates to the "Dark Kingdom" of the latter to connect the stories. The final enemy in Codename mentions the war between the Moon and Earth kingdoms which is the main plot of the BSSM manga. Furthermore, there exist mysterious male characters in both that draw the attention of the main female protagonists: Kaitou Ace in V! and Tuxedo Kamen in BSSM. Also, the crescent moon on SailorV's forehead plays a significant role in the later acts of the Dark Kingdom Arc. Yea, it seems like Naoko was unoriginal, but she wasn't. Naoko just wanted a new heroine and story for the anime production of her manga - something she could expand on.
BSSM has evolved from this humble manga collection to an international anime. It has been translated and shown in Mexico, France, China, Australia, US, and many other nations. Though the public criticizes the anime for not "staying true" to the manga, it nevertheless introduced Sailormoon to the world and started a major interest in anime and manga from Japan. Don't think BSSM was the first anime or manga to hit big in other countries! Rather, it's one of the more noticeable ones.
For more information on the history of BSSM, please visit The Moon Garden or MangaStyle.