A layout is a collection of map elements laid out and organized on a page. Common map elements include one or more data frames (each containing an ordered set of map layers), a scale bar, north arrow, map title, descriptive text, and a symbol legend.
Layout view is where you add map surrounds, frames, graticules, and other finishing touches to a map. What you see on the layout is what you get if you print or export the map to the same page size.