A book is a book… Right? The most obvious differentiation between types of books is that between a novel and a picture book. "Picture" book not only denotes the medium you’ll find within and the intended audience, but the expected length. If a book with pictures has an educational purpose and a thicker page count, it gets upgraded and referred to as a textbook. What many people don’t know is that fiction works within a system of categorization too: What is a novella, and when does it become a novel?
A novel is most plainly described as a work of literature that is fictitious and of book length. This definition excludes non-fiction, as a narrative work about a real-life person would fall into the genre of memoir or biography, but the confusing part is the specification that a novel is "of book length." What does that mean?
"What is book length?" is a self-defining question. The bare-bones definition of "a book" is a work that’s been written or printed on pages that have been glued or sewn together along one side, bound by a cover. A book does not have a mandatory page count.
As described above, in mentioning a "picture book" it is the genre of the story within that determines the expected length of the book. Therefore, although a novel is "of book length," it’s a bit of an oxymoron. A novel is book length insofar as a book of another type is not a novel, based on its length.
When someone says they read a book, it connotes a length, but it is calling it a novel (or elsewise) that denotes the page count.
The length of a novel can be anywhere from 60,000-100,000 words, the average being 80,000. Works as short as 50,000 words can be considered novels, given their genre provides sufficient content for so few words, but that’s the minimum most will accept.
It is easier to define a novel (or novella) by word count rather than page count since there are many ways to manufacture or minimize the pages in a book: font scale, formatting, interior design elements, trim size. Although some may say that anything over 100 pages can be considered a novel, that doesn’t account for genres like prose collections or narrative poems.
Anything that is still a fictitious narrative, but under 40,000 words is considered a novella. A novella is "a novel" that doesn’t reach the expected word count. They are referred to as a "short novel" or even a "long short story." The category of book below novellas in word count are novelettes and short stories, respectively.
Since defining a novel from a novella doesn't serve to comment on anything other than their word count, why bother differentiating them at all? Although a novella does not inherently lack narrative structure, its compactness can often lead to electively excluding more elements than a fully fledged novel.
The genre of the story also plays a large part in reader expectations and what they are willing to accept. For example, a scifi novel and a scifi novella should supply different experiences. A scifi novel should have a lot of worldbuilding, a variety of moving plots, and character development. If you read a 150,000 word scifi novel and felt as though the environment was never fleshed out, that would be a major criticism. A scifi novella, on the other hand, could acceptably leave a lot of questions unanswered about the world in which it takes place. A novella could have a small cast that only ever exists in a single scene, and readers would be satisfied in that minimalism so long as the reduced fluff served to enhance the core narrative or concept.
A good way to think about it is that a novella is not "less" story than a novel, it is a distillation—a condensing—of the delivery.
After learning about what makes novellas unique (even preferable to many), and you’re interested in the shorter side of fiction, here’s an article on $ How to Write a Short Story with an Outline$ —it’s even got example templates to get you started!