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A window is two views: the frame view (which contains controls that only AppKit has access to, like the zoom, hide, close, and maximize buttons, and the title bar and proxy icon), and the content view, which you control.
If you're designing an area you're going to embed within another view, or hand off to an API that wants a view (for example, to create a panel or sheet), then use a view. If you're designing something to specifically be a window, like a document window, then use a window.
With a window, you have additional abilities to control its appearance and behavior, for example, to make it modal, give it a proxy icon, list it in the Windows menu, order it relative to other windows, etc. that you can't do with a plain view.