In its review of Josh Weil’s 3 novellas (wrapped up into one volume), the NYT gets to the heart of what a novella can be:
A good novella has an intensity and concentration rarely found in novels, and an expansiveness and scope rarely found in stories. If a short story is a piece of furniture and a novel is a house, then a novella is a room — and in that room a skilled writer can sometimes find space for all the aberrations and terrors and longings of a character’s life. The right room can intimate its occupant’s past and future, frustrations and failures, the shape of the house beyond.
The room is a great metaphor for the novella. On a smaller scale than the novel, the novella is cozy. Each word must be carefully chosen — I’ve spent half an hour just crunching out one sentence — and because of this the detailing can be that much more rich and fluid.
The room analogy, however, also makes the novella seem like a subset of a novel (a room is a subset of a house), which it is not. Character development and complexity must be as fine-tuned as a novel. This may in fact make novellas trickier to write because the action and description must convey the same amount of understanding as a novel, but in the confines of a much smaller, more intimate space.