Pretty Soldier Sailormoon originated as a manga (see: japanese comic) series written by Takeuchi Naoko in 1992. It was originally run in Nakayoshi magazine which is mainly for shoujo, or girls, stories. The story was then published in compilations called tankobon (see: graphic novels) by Kodansha Comics. The series was then picked up by TOEI Animation who decided they wanted to turn this sailor suited girl warrior manga into an anime (see: animated) series to run on TV.
Pretty Soldier Sailormoon? Bishoujo Senshi Seeraamuun? What is the difference?
The romanization of the manga title is Bishoujo Senshi Seramun which roughly translates to beautiful girl soldier sailormoon. The most widely accepted translation and the name that appears on the merchandise is Pretty Soldier Sailormoon. The translation above is just a break down of the words 'bi' and 'shoujo'. Bi meaning beauty or beautiful and Shoujo meaning girl.
Why do I sometimes see BSSM or PSSM?
These are the abbreviations for Bishoujo Senshi Seramun and Pretty Soldier Sailormoon. Sometimes it is much easier to write out that abbreviation instead of the whole title.