A Viewport isn't a layout strictly speaking. It is a special type of container to represent the entire viewable area (the viewport) of the web browser window.
On a web page, the Viewport sits at the top-most level, rendering to the document body (or to the ASP.NET form if it is present on the page). From there it can automatically size itself to fill the browser viewport and resize when the browser window size changes. Therefore, there is only one Viewport in a page.
A benefit of the Viewport is that you can contain the whole user interface of the application inside this Viewport. Unlike a traditional web page where you may scroll beyond the page, the Viewport makes your application look more like, well, an application! You never scroll beyond the page (components contained inside a Viewport, of course, can have their own scrolling if needed). This means, for example, you can have a fixed menu or toolbar at the top that will always remain there, or if you have buttons at the bottom...