The Magic Knights of Rayearth (MKR) was a very popular shoujo series in Japan by Studio CLAMP. Shoujo is the Japanese term for a series aimed primarily at young girls. This guarantees the series will have:
The series started out as a manga but it became popular enough to have its own OAV and TV series. The series was based on the manga but the OAV took the characters and put them in a different plot. MKR's success was based on combining popular elements from other series (Magical girls, giant mechas, Fantasy, Action, and SD humor) and adding a story filled with romance, character development and twists.
The first season of the TV series is fairly simple and true to the original manga. Three middle school girls, Hikaru Shidou, Umi Ryuzaki, and Fuu Houji are summoned by the Princess Emeraude of Cefiro to become the Magic Knights of Rayearth and save Cefiro. This, however, wasn't in the three strangers day planners and they are a little surprised at being ripped away from their classmates at the Tokyo Tower and hurled into Cefiro.
Nonetheless, they manage to make friends with each other, meet Master Clef and get equipped to fight in about 10 minutes. Then, poor Clef gets trashed before he can explain everything (What fun would it be if you knew everything!) and they're on their own. The show follows the heroines as they grow personally and become the Magic Knights of Rayearth. The animation quality is decent and the super deformed humor segments are fun to watch.
This isn't the most cerebral of series. Frankly, the show made my brain shut off after the first 15 minutes of watching it. I was busy being sick at the time so I didn't mind. However, the characters are quite well developed and humorous. Both, Ferio and Mokona in particular amused me.
The ending of the first season makes watching the show worth while. It is quite a profound ending for a show aimed at young girls. The ending is quite tragic and romantic. *sigh*
The opening theme song, Yuzurenai Negai, is quite nice and I had it stuck in my head for a long time after I watched the series over my Christmas break. Now, I think of Christmas when I hear it. The second opening called Hikari to Kage wo Dakishimeta Mama by Naomi Tamura (Still holding the light and darkness) was also a good choice.
Lets make this clear. The show's target audience is teenaged girls. I whole heartedly don't recommended it to anybody else. In fact, I'm not quite sure how I managed to survive my marathon viewing of the first season of the TV show. I must have been a bit mentally sick as well as physically sick when I watched it.
The OAV series, Rayearth, is probably the best overall animation adaption of the manga. It is considerably darker but just don't expect it to have much of a connection with the series or manga. I don't really recommend the second season of the TV show. It lacks much of the charm the first season has.
One of my favorite online resource for Japanese is Reiko-chan's Japanese For Anime Lovers. It uses sound clips from MKR to teach basic japanese phrases and grammar. Its worth checking out if you're interested in learning Japanese.