Outline vs Structure
An outline is a list: the sequence of events in a novel, set down before the writing starts or reconstructed afterward to see what actually happened on the page. Structure is something else, sitting underneath that list rather than beside it. It's the shape the events make when they're read together, the pattern by which one scene sets up a change that the next scene has to answer. An outline can be perfectly detailed and still describe a book with no structure at all, because listing events in order says nothing about whether each one moves anything forward.
Where the confusion comes from
The two get used as synonyms constantly, and the mix-up is easy to understand: both describe the same material, just at different resolutions. A plan lists what happens. A structure holds, the way a building holds, because each part is doing load-bearing work for the ones around it. A writer can hand over forty outline bullet points and still be unable to say what the middle of the book is actually about, because the list never asked that question.
How this shows up in a real novel
Jane Eyre (Charlotte Brontë, 1847) has a simple outline: a girl moves through five places in turn. Its structure is what those five moves actually do to her. Each location takes something specific away or hands something back, so the sequence reads less like an itinerary and more like an argument being built one stage at a time, ending somewhere the opening chapters never could have.
While you're drafting
Most writers only find out which one they're missing partway through, when the middle of the manuscript keeps producing scenes without producing any sense that something is actually changing.
Questions
Is structure just a fancier word for an outline? Not quite. An outline lists what happens, in order. Structure describes what those events do to the reader and the protagonist as they accumulate, which is a separate question from the order itself.
Can a novel have a detailed outline and no real structure? Yes. A list of forty scenes can still leave a reader unable to say what changed between scene one and scene thirty, if none of the scenes actually shifted anything for anyone in them.
Do you need an outline before structure can exist? No. Plenty of structure gets found after a full draft already exists, in revision, by looking for the places where something changed and the places where, on closer look, nothing really did.