Chibi Usa was first seen in the Sailormoon R series. She dropped out of the sky when Usagi and Mamoru were on a date. She then attempted to take Usagi's ginzuishou (imperium silver crystal) by holding a toy gun to her head, and since then, the two have never really been on each other's good side. Chibi Usa is in fact, the daughter of Sailor Moon. In the 30th century, the city of Tokyo was known as Crystal Tokyo. When the Dark Moon Kingdom attacked Crystal Tokyo, Queen Serenity (the future of Sailor Moon) was injured, encased in layers of crystal, and put into a deep sleep. Chibi Usa then went to the past to request for help from Sailor Moon, whom she had heard her father, King Endymion (the future of Tuxedo Mask), declare the strongest senshi of all time.
One point I'd like to make a note of, is how the American DUB has portrayed Chibi Usa's relationship with Usagi. Admittedly, Chibi Usa is a brat. There's no arguing that. But the American DUB has turned her into a monster. The relationship she has with Usagi isn't one that consists entirely of arguments, heated fights over Mamoru, yelling, screeching... you get the picture. In the manga, as well as the Japanese anime, there is an understanding between the two that one may either conclude borders on a sibling-type relationship, or a mother-daughter relationship. In either case, an occasional disagreement between the two is to be expected. In the end, the fact remains that Chibi Usa is Usagi's future daughter, and that Usagi is Chibi Usa's future mother. No matter how the American DUB has changed their relationship to suit their needs, the two care and love each other very much. So please, the next time you watch an episode where the two are at each other again, remember that Chibi Usa may be a brat, but she isn't a monster. ^-^;;