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Quick Answer:

A progressive web application (PWA) is a hybrid internet revolution. This new web technology enhances UX giving the user more opportunity. This innovation was created by taking the best bits of a native app and Web App, in essence creating the best of both worlds.

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Technical answer:

For a PWA to work there a two elements which are needed, The Service Worker and The Web App Manifest.

• The Service Worker is vital to allow a PWA to function offline. What it is, is a JavaScript background task that replaces the traditional web cache by using a cache API. The cache API wakes up when needed, listening to different network requests saving the static resources from the visited sites. The Service Worker saves these resources by redirecting the web page from going to the server but instead go to The Service Worker.
• The second element which a PWA needs is the Web App Manifest, this is a JSON script, making it possible for the Web App to have a home screen icon, theme, splash screen and the ability for the web app to run in full-screen mode and as a standalone application.

PWA’s can be viewed as a set of web pages or a single web page that are rendered on a mobile device browser. They use common web languages such as HTML, HTML5, CSS, etc which simulate the retrieval of data from a mock API, all in a shell like architecture. To give the application it’s native appearance, a the Web App Manifest is used to describe the application and the Service Worker is used similar to the cache.

A common analogy for how a PWA works is related to the M&M peanut simulating a self-contained unit.