Summary:
Magic Knight Rayearth is set in two worlds: Earth and the magical world of Cephiro, where a person's strength of will determines all. Three earth girls, Hikaru (red hair), Fuu (blonde/brown hair), and Umi (blue hair) are transported to the world of Cephiro. In that world, the girls must become Knights and harness their own special powers in order to help save the land, though the toll it takes on them may be too much...
The manga spans 6 graphic novels with the first 3 being a complete tale (Magic Knight Rayearth I), and the second 3 being another tale (Magic Knight Rayearth II).
The anime is 20 episodes long for the first season and covers the first three manga, and 29 episodes long for the second season. The second "season" of manga is a bit different than the second season of anime.
All in all, I prefer both the second season of the manga and anime to the first (the first is a bit "monster of the week"). Though the art for Magic Knight Rayearth is quite cute, the series itself can be a real tear-jerker. There's a bit of death and angst in this otherwise empowering "magical girl" story. Though more for the younger set, I still really enjoy the Rayearth manga to this day.
There is also an OAV which is a retelling of the story, with some changes. I have not seen the OAV so can't really comment at this time.

Best For:
Ages 10 and up.

Contains:
Action, Fantasy, Drama, Romance

Read it: You can buy the English-translated manga (note that this edition is unflipped! Yay!):
Magic Knight Rayearth I: Book 1, Book 2, and Book 3.
Magic Knight Rayearth II: Book 1, Book 2, and Book 3.
Additionally, if you prefer your manga in pretty box sets, you can also purchase
Magic Knight: Rayearth I (Boxed Set) and
Magic Knight: Rayearth II (Boxed Set).

Admire it: You can purchase the English-translated artbooks, The Art of Magic Knight Rayearth Vol. 1 and
The Art of Magic Knight Rayearth Vol. 2.

Watch it: You can buy the anime Magic Knights Rayearth Economy Box 1(season 1) REALLY frickin' cheap!
Magic Knights Rayearth Economy Box II (season 2, also frickin' cheap) will be released in late August,
so you can preorder now!