What is a graphic novel?
Graphic novels incorporate any type of narrative with sequential art, including art in comic strip form and differentiates from comic books by being bound in soft- or hard-cover and containing substantially more material and pages.
What is manga?
(漫画, or "comics" or "whimsical drawings")
is a subset of graphic novel that is created in Japan, or by Japanese authors, in the Japanese language and/or of to the style developed in Japan.
It features a variety of genres (action-adventure, romance, sports and games, historical drama, comedy, science fiction and fantasy, mystery, horror, etc.)
for a variety of age groups (young boys, young girls, school age boys or girls, adult women, adult men, etc.).
Like graphic novels, manga artists have their own style, and draw characters according to that style, so characters and storylines vary.