漫画, Manga** is the term usually used for Japanese comics or graphic novels. Outside of Japan it refers specifically to comics originally published in Japan, or works similar in style to Japanese comics. Native Japanese are often unaware of the use of manga to refer to Japanese comics specifically.
The drawers of these comics are called mangaka (漫画家).
Manga currently represents a multi-billion dollar global market.
Manga are read by almost all Japanese social classes and age groups. Themes include sports, romance, historical drama, comedy, soap operas, fantasy, mystery, sexuality and horror.
Manga is almost always in black and white, except for the covers and often the first few pages; in some Animanga (anime printed in manga style) all the pages are colored. They usually originate in Japan, and sometimes China, or other East Asian countries.
Very popular manga are sometimes adapted into anime (Japanese animation - basically just animated manga) once a market interest has been established. Adapted stories are often modified to appeal to a more mainstream (and often more conservative) market. Original anime is also sometimes adapted into manga, but this is unusual.
There is a popular misconception that the term "manga" refers to Japanese animation, but actually only "anime" (アニメ) is the correct term for this.

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Tohru Honda from Fruits Basket, and Edward Elric from Fullmetal Alchemist, both examples of typical manga/anime drawing style.
Fruits Basket and Fullmetal Alchemist are good examples of Shoujo 少女(girl) and Shounen 少年(boy) manga respectively.