Sera Myu is one of the most popular musical acts, and began production in 1993, although as of 2005 they have gone on hiatus. The musicals were usually performed during two seasons of the year, and had a rotating cast, older members some times coming back for reunion specials.
Costumes for the Myu are a lot more ornate, and characterizations are sometimes a little different to standard, and sometimes actors, particularly villains will recycle for various roles throughout the series.
The Myu is divided into four stages so far. A new stage counting when the actress playing Sailormoon is changed. While containing characters who are familiar from the anime and manga series the musicals do not strictly follow the same plot lines, and in some aspects personalities of the familiar characters are different to what you might expect also. They do follow the same plot arc as the anime and manga series, each arc ending with the appearance of the Starlights and fights with Galaxia.
More input on Myu will come once I'm able to watch some with translation to know exactly what is going on...