Easy answer: http://en.wikipedia.org/wiki/Light_novel
Why Did You Choose This Format?
I chose the light novel format because I like the combination of story interspersed with manga-style illustrations. I hope it becomes mainstream in the future, because it’s a wonderful medium once you get used to your books having pictures in them! For more on the rising popularity of the light novel format here in the USA, and the rise of English-language light novel authors, check out my post, In Defense of the Original English-Language Light Novel.
Rocket Girls was the series that led me to discover light novels, as the anime was originally created from one. Since then I found out that many of the series I enjoy now were adapted from these novels—which often started out being self-published! That, combined with the recent revamp of the Sailor Moon franchise got me to thinking about a premise I came up with in high school and made me wonder if I could retool it into a plausible, enjoyable story. Hopefully I have done this with my own light novels!