Responsive Web Application is an approach of laying-out and coding a Application such that the Application provides an optimal viewing experience.
For web apps you expect even more extensive user interface capabilities than on a web site meant for reading. On a tablet, for example, you would expect, as appropriate, a mixture of scrolling lists, toolbars, choosers, keypads, and other UI widgets as well as popups, dropdowns, and slide-in tool trays to hold them. When you rotate a device from portrait to landscape you would expect the layout to change, perhaps moving a toolbar or list from the top or bottom to the side, and perhaps changing the direction you need to swipe or scroll. When the same app runs on the smaller screen of a smartphone, you might expect some of the areas to move off-screen with handles or buttons to bring them into view, and the contents of listings to be terser, with details possibly being available by tapping on a row.