Uranus is the seventh planet in the Sol system, between Saturn and Neptune. It is a gaseous ice giant and the only Sol planet which rotates on its side in relation to the rest of the system. This causes the planet to have vastly different seasonal changes than most of the planets in the system.
Uranus, like several other Sol planets, has rings and 18 moons. Most of them are named for characters in the works of William Shakespeare. The element uranium was named in support of the name Uranus for this planet.
In Greek mythology, Uranus was god of the sky, the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Some aspects of this lineage can be seen in Sailor Moon, where Ten'ou Haruka and Tomoe Hotaru have a father-daughter type relationship, and Kino Makoto often looks up to Haruka. Also accordingly, Sailor Uranus is the senshi of wind and is often related to the sky, as seen in the kanji for her last name, 天王 (Ten'ou).
More information about Uranus can be found on its wikipedia page.