If it weren't for the characters, we wouldn't have the show no da.  The senshi are the most important ones, well they would be, seeing as they're shown the most. Villains are important too, don't get me wrong, I like them too.  But this page is about the senshi so...hahaha. :p
Inner Senshi
The inner senshi were the first ones to appear in the series.  Their mission was to protect the princess of the moon (which is later revealed to be Sailormoon/Usagi), and in the Silver Millenium they were Princess Serenity's closest friends.  Except for Chibiusa, who is really Usagi's daughter from the future in Crystal Tokyo, Usagi Small Lady Serenity.  Click on a name to learn about that character:
Sailor Moon/Tsukino Usagi
Sailor Chibimoon/Tsukino Chibiusa
Sailor Mercury/Mizuno Ami
Sailor Mars/Hino Rei
Sailor Jupiter/Kino Makoto
Sailor Venus/Aino Minako
SD Inners in Super form, Eternal Sailormoon
Outer Senshi
The mission of the outer senshi was to protect the Moon Kingdom.  Most of them first appeared in SMS, when Uranus and Neptune were searching for the three Talismans that make up the Holy Grail. They are more powerful than the inner senshi, because if the solar system were ever attacked the outer planets would be hit first, so their defenders needed to be more powerful. Okay, like above, click a name for a profile of that character:
Sailor Pluto/Meioh Setsuna
Sailor Saturn/Tomoe Hotaru
Sailor Neptune/Kaioh Michiru
Sailor Uranus/Tenoh Haruka
manga Outers, neat picture I think