Scrivener is a content generation tool for writers. Word is a word processor.
Scrivener started out as a Mac program, Word as a Windows program, so users used to divide along the lines Windows users used Word while Mac users used Scrivener (or a Mac word processor). Nowadays, both programs work for both types of operating systems, so it’s more a case of preference.
Many of the things you do in one program can be done in the other, even if not the same way.
Scrivener was designed for novel writing, while Word is a general purpose word processor. You can write letters or reports on it just as easily as you can write novels. So I imagine that many novelists would find Scrivener useful and easy to use. Especially if you’re a plotter. Almost certainly if you already use index cards or similar to plan out your stories.
Me, I use Word. I’m a Word guru from way back, a Microsoft user all the way. I’m also a pantser—or as Brandon Sanderson calls it, a gardener (he borrowed the term from George R. R. Martin). I’m happy to let my words flow. I go back and rewrite along the way, and move big chunks of text around while I do it. I like having the whole book in one document.
My co-author, Sherylyn, uses both. Scrivener when she’s planning out a story, or writing non-linearly, Word when she’s just letting the story flow.
Neither program is necessarily better than the other. Both have good and bad points. It’s what works for you and the way you write.