What does a novel's outline look like?
Well, kind of a rectangle, I'd say.
...What?
But seriously, folks... Basically an outline is a chapter-by-chapter or scene-by-scene summary of the plot. It's basically the equivalent of what's called the "story" in film and television. It's how a writer works out what will happen in the tale, what the character arcs will be, etc., so there's a road map to follow for the actual writing, and so that you know that what you're writing will hold together and make sense as a whole. (I tried writing my latest spec novel with only a rough outline that I diverged from radically in manuscript without bothering to re-outline first, just trying to figure it out as I went, and that took me down a dead end. I wasn't able to get back on track until I thought through the whole thing more carefully and put together a new outline.) For tie-in work, it's additionally part of the approval process -- you have to get the full plot outline approved before you proceed to manuscript.
What it actually looks like depends on the writer. Some of us write fairly loose outlines, while others go into deeper detail. For instance, Dave Mack, influenced by his screenwriting background, works out intricately detailed chapter-by-chapter, scene-by-scene breakdowns of everything that's going to happen. I tend to be a little looser, and there are generally details I don't discover or figure out until the actual writing, but I'd say I'm about midrange in my level of detail, compared to other authors' outlines that I've seen.