Form is like the format of a letter. If you look this up on Google, you will see things like heading, address, salutation, body, and signature. The format does not tell you what to put in each of these places. It only tells you the kind of information that goes there.
Form in literature is the format of the piece of writing. The word novel gives you information such as the fact that it is fiction. It gives you a relative idea of length. It also tells you that there will be a long, slow build to the climax of the story. The conflict of the story will be resolved, and it will have an ending. The word novel doesn't tell you much more than that. Form is the container the story is poured into. Genre is the story.