A manga is a Japanese or Asian style comic book. it is read from right to left and are inked in gray scale. Many manga's are also lengthier than a typical "Western" style comic book.
A graphic novel is a stand alone work or a collection of comics in a single work that encompasses a single or entire story arc. Graphic novels are also lengthy, usually in color, and the stories may be more in depth or have greater intricacy than compared to a comic book. The story is told using sequential art and text.
Comic books are published on a periodic basis (i.e. monthly, weekly, etc.), are usually inked in color, tells a story using sequential art and text, and are short in nature-40 pages or less.