Soldiers of the Four Guardian Deities

Soldiers of the Four Guardian Deities is the official name given to the team of the four sailors Mercury, Mars, Jupiter and their leader, Venus. The full title is rarely used, and is usually shortened to the "Guardian Senshi". Fans often use the title "Inner Senshi" to refer to the group.

Information
During the age of the Silver Millennium, the Inner Senshi were the guardians of Princess Serenity. They were also the princesses of their respective planets, but lived on the Moon at times. When Queen Metaria and the Dark Kingdom invaded the Moon Kingdom, the four were killed while trying to protect their princess. After Queen Serenity sealed away Metallia, she used the power of the Silver Crystal to send the Inner Senshi, Princess Serenity, and Prince Endymion to the future to be reborn on Earth to live happy lives.

In 20th Century Tokyo (21st Century in the new anime), the Inner Senshi were reborn as normal human girls. However, when the threat of the Dark Kingdom arose, they were forced to awaken their powers. Princess Serenity's reincarnation, Usagi Tsukino (Sailor Moon) discovered the reincarnations of her four guardians, who would soon become her dearest friends: Ami Mizuno (Sailor Mercury), Rei Hino (Sailor Mars), Makoto Kino (Sailor Jupiter), and Minako Aino (Sailor Venus).

In the manga and Crystal, Sailor Venus was the leader of the Inner Senshi and would impersonate her princess (since she resembled her the most), Serenity/Sailor Moon to keep her from harm.

In the time of Crystal Tokyo, when Usagi becomes Neo-Queen Serenity, the Inner Senshi still act as her guardians and live in the Crystal Palace with her and the rest of the royal family.

When there isn't a threat going on, the Inner Senshi are more connected to their human lives; dreaming about their futures (despite already knowing them), and being regular teenage girls. However, when they're needed as Senshi, they are ready to protect their world and their princess from harm.

Trivia

 * Minako was the first of Inner Senshi to be awakened, as Sailor V instead of her true form as Sailor Venus. Though after she defeats the Dark Agency, she becomes Venus and starts her quest to find her comrades.
 * She is also the only one of the Inner Senshi to have been awakened by Artemis instead of Luna.