From Sailor Moon Flash!
The starseed is essentially a soul in concrete form. All living beings have starseeds, animals as well as people; removing a starseed allows Chaos to take its place, and turns a human into a big scary monster until such time as it is purified and returned.
However, some people have true starseeds, often referred to as Sailor Crystals. They shine more brightly than an average person's starseed, and can remain apart from its host eternally (the host, however, does not much enjoy this). These bestow special powers upon the holder and enable the hosts to be soldiers in the war against chaos and the forces of evil. The starseeds choose their own hosts after their formation in the Galaxy Cauldron; it is assumed that they move from generation to generation, selecting new hosts as time passes.
Aside from the specific, area-focused power of the Sailor Crystals, the rest of the fundamental power of the universe is split into four pieces: the Great Crystals, Silver; Gold; Ruby; Sapphire. The wielders of these can call upon the power of their starseeds in an explosion of power known as an Onegai.
