Generalizing a little bit, manga is basically a Japanese comic book. In the English-speaking world as a generic term for all comic books and graphic novels that were originally published in Japan. There is the term Amerimanga for woks that are develop by Americans in the manga style.Manga can also refer to publications made in a montly magazine; however, I'll refer to manga as graphic novels or comics. A manga graphic novel can consist of a couple of volumes to more than 20. Also, another characteristic that makes Manga popular is the fact that there tons of publications, and there's one for anyone; for example, one can contain a deep political message, and other can be just a simple superhero story. There are a couple of distinctive characteristics of it. The draws are usually made in pen and ink, emphasizing the use of clean lines. Also, the eyes have a distinctive shape (I'll be easier if I just show it).
Source: http://www.wisegeek.com/what-is-manga.htm
