Magic Knight Rayearth and Magic Knight Rayearth II were two three-volume manga serials written by CLAMP, a four-woman artist and writer team. The manga was then followed by a television series which faithfully animated the first series and was directed by Hirano Toshiro. The television show was quite successful and was followed by another television series that was based on Rayearth II but diverged somewhat from the manga's storyline. After the television run ended, Hirano still wanted to do something with the Magic Knight Rayearth concept and characters. This resulted in the OAVs, which retained the title Rayearth because of the characters and basic artistic design but were unrelated to the storyline written by CLAMP. However, to better appreciate the OAV series, one should understand a little about the three main characters' attitudes.
Shidou Hikaru: She is a very energetic red-haired girl. Hikaru is brave and has a very positive attitude.
Ryuuzaki Umi: She is a very loud and self-confident blue-haired girl. Umi never deals well with orders from others.
Hououji Fuu: She is a very kind and smart brown-haired girl. Fuu loves to point out the blazingly obvious.
Part of the allure of the OAV series is contrasting the personalities and motivations of the characters here with those in the TV series. At the start of the TV series, the three girls do not know each other and all go to different schools. Their friendship is developed over time within their adventure in Cephiro. In the OAVs, the girls are all friends from the start and all attend the same school, the elite CLAMP School (also home to CLAMP Campus Detectives). Interestingly enough, the fact that the girls go to the CLAMP School clearly places the OAV series outside of the canonical CLAMP universe, since they have stated that the Rayearth series is definitely in a totally different universe from the one that many of their other works, such as CLAMP Campus Detectives, X, Tokyo Babylon, and Dukylon, reside in. Additionally, the characters from Cephiro are supposed to be familiar to the audience from reading the manga or watching the TV series, yet many have different roles and motivations.
If you're interested in the storyline of the original TV and manga series, point a web browser to: http://www.anime.net/~fuu/cephiro.html
Here are some random tidbits. Each girl is associated with an element, which is reflected in her name: Hikaru (Light) is associated with fire, Umi (Sea) is associated with water, and Fuu (Wind) is associated with air. Mokona is named after a member of CLAMP, Mokona Apapa. Almost all of the characters from Cephiro are named after cars. Some, like Eagle Vision, are obvious to the American viewer while others, like Ferio, are more obscure, since many familiar American cars have different names in Japan.