When a series is marked as a novel, what does it mean?

For example: Zero no Tsukaima (novel).

Does that mean that it's not really a manga?

Exactly, it's the light novel that is being translated. In most cases the light novel is the original and the manga is an adaptation.

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This week's favorites:

- Golden Kamui





ççççççç[Ô .Ô] tsutopodus© will eat your manga and steal your cats!

Some info on light novels:

Light novels are the "next big thing" in the anime/manga world. They're novels that contain manga-style pictures, some color pictures for the first few pages and a least one black and white between each chapter. Light novels usually come before the anime/manga adaptation of a series, but the Karin novels are a spinoff of the original manga.
Some popular light novels are:

Suzumiya Haruhi no Yuutsu
Zero no Tsukaima
Shakugan no Shana
Shinigami no Ballad
Strawberry Panic
The Twelve Kingdoms
Karin

English translated light novels are now easier to find. Companies such as Tokyopop, Viz, Seven Seas and Yen Press are currently releasing some light novels. Try one some time! ^_^