"Those shooting stars are the invaders. They have come to Earth as stars." - Haruka, Act 45
To understand the Stars arc of the manga, you'll need to know some important terminology. Fortunately, the Kinmokusei senshi were kind enough to explain these terms to Usagi, so I've summarized it here:
Every spark of life in the universe originates from star seeds. They are the essence of life and creation - even planets have star seeds. Sailor crystals are special star seeds, born interconnected with planets, and grow together with the planet; these crystals are incarnate in sailor senshi. A sailor crystal is a senshi's heart and soul, and the source of the senshi's power.
Unfortunately, this power can be stolen. And when a senshi's sailor crystal is taken, that senshi's very life is taken along with it. When the crystal is gone, the heart and soul are gone along with it - and the body is destroyed.
The Stars arc begins with a power-hungry senshi named Galaxia, who is determined to gather all of the sailor crystals in the galaxy into the Galaxy Cauldron. Whenever she acquires a sailor crystal, she acquires the power inherent within it, and is able to distribute the power among her servants, the Anima Maids. One by one, she gleefully seizes the sailor crystals of Usagi's friends, leaving Usagi more and more alone.
However, although she is powerful and ruthless as she is, Galaxia is not the real enemy. The power behind this attack is Chaos, who is even more power-hungry than his metal-clad pawn.
The Stars arc of the manga has a definite feeling of finality about it. Although events to come are hinted at, you get the impression that Takeuchi-san is done with the series. This is possibly one of the most decisive endings I've ever encountered - all the major loose ends are tied up. Ultimate good has triumphed over ultimate evil, the girl gets her man, and everyone lives happily ever after.