SPA is a web app that fits on a single web page that provides a fluid user experience. A SPA is fully (or close) loaded on the initial page load, it’s key resources are preloaded, and progressively downloads features as required. A SPA may also persist and maintain important state on the client while also handling navigation, history and deep linking. A great examples of a SPA is gmail.
The user experience is key. If you can speed up the perception of page loads and navigation for the user, you win. A SPA preloads what the user may most oft use, so when they navigate they are not making network trips and page reloads on those oft used features. Thus the result is a snappy app and a happy user.
One of the goals was to make it work in mobile device browsers. So thanks to the great tool known as Electric Plum, you can see the screen shots here from an iPhone and iPad. I used a few other testing platforms too that I go into in the Pluralsight course.