In the west, Manga are defined as Comic Books of Japanese origin. As a medium, there isn't a difference between Manga and American comic books because they both follow them same method of sequential art. However, there are cultural differences in the way they are presented to the consumer.
In Japan, various Manga stories are printed in anthology magazines (like the Shonen Jump) on cheap paper with no color. Eventually, the individual stories will be reissued in their own digest style collections and resold directly to the consumer.
On the other hand English language comic books are usually given their own individual pamplet that includes color and are reprinted in trade paperback forms.
In Japan, comic books are very popular (the most popular mangas, like One Piece and Dragonball, often sell into the millions) and are more respected as an art because of the various genres that are on the market. While English comic books have a very limited readership and are still looked down upon by many as a worthless art-form and mos of the comics found in comic shops derive from the superhero market. One week sales of a manga in japan outsells the entire yearly American comics market.
The vast majority of Manga's success can be traced to Osama Tezuka, the man often considered to be the father of Japanese comics. In his lifetime he created Astro Boy, the Buddha series, and the Phoenix collections (which is often considered by critics as his defining work).
Popular Manga: Dragonball, Naruto, Berserk, Akira.
Manga is a legitimate artistic medium.