Why are novella's not novels?
Novella
A novella is a text of written, fictional, narrative prose normally longer than a short story but shorter than a novel, somewhere between 17,500 and 40,000 words.
Graphic novel
A graphic novel is a book made up of comics content. Although the word "novel" normally refers to long fictional works, the term "graphic novel" is applied broadly and includes fiction, non-fiction, and anthologized work. It is distinguished from the term "comic book", which is generally used for comics periodicals.