When I picture genre, it is all encompassing. It includes what the characters are likely to be like, the plot, the setting, the writing style, the length of the book, the target demographic, the themes, and the list goes on. Any aspect of a book that can be written down to a formula adds to the overall genre. Let’s analyze the latest fading trend: young adult dystopian trilogies. Off the top of my head I can name three young adult dystopian trilogies from when I actively read young adult literature. How is that possible? Not only young adult dystopian book series, but series that have specifically three books. The one that started the trend was The Hunger Games. A teenager who will do anything for their family (specifically a younger sibling) stands against the entire government and wins. Add in the love triangle and you have yourself a bestseller.
Form, on the other hand, is the physical format of the information. It’s something that can apply to many genres. For example, you can have short stories as the literal form of the writing but the genre could be nonfiction, fiction, mystery, crime, suspense, horror, the list goes on. When you have all the materials, you can go down many different avenues. When you start specifically with the genre, you eliminate dozens of forms of writing because it just doesn’t fit. Genre and form usually need to harmonize. You can’t really smash recipes and horror together, it’d be complete bullshit.