Manga is a Japanese word for comics. Many manga books either follow or begin an anime series and contain the characters in the anime. Still other manga books contain new characters and situations. Manga also has a large variety of types and audiences, from children to adults.
All Japanese manga is written in the Japanese language, if you can't read japanese you can at least try and follow allong with the pictures. And if you try hard enough you can accurately guess what is being said.
I myself have an impressive amount of manga. Though I collect Aa Megami-sama! and Tenchi Muyo! Ryo-ohki manga.