In terms of art style, cartoons can be very simplistic with bold lines and bright colors for easy recognition. Graphic novels can have a wide range of art styles from highly detailed realistic to highly stylized. The art in a graphic novel is also used to enhance the mood and depth of the story. It's not just for visual appeal but an integral part of the storytelling process.
A cartoon is often short and simple, usually presented in a serialized form like in newspapers or short animations. For example, a daily comic strip. A graphic novel, on the other hand, is a complete, longer story in a book format. It has a more complex narrative structure and can cover deeper themes.