Neptune is the eighth planet in the Sol system, between Uranus and the Kuiper Belt, of which the largest member is 134340 Pluto. It is considered an ice giant, and in some ways a twin planet to Uranus. Because of its gaseous composition, the equator and poles of the planet have different rotational periods, and between the planet and it's magnetic field. Neptune is a ringed planet and has 13 moons.
Neptune is named for the Roman god of the sea, and its symbol is a stylized trident. Sailor Neptune's attacks are seawater based, and her fuku is sea green.
More information about Neptune is available on its wikipedia page.