Progressive Web Applications (PWA) is a type of web application that can be described as reliable, fast, and engaging. Moreover, it must implement such technologies as Service Worker, Web App manifest, HTTPS, App Shell, and Push Notification.
In other words, PWA is a web application, oriented to deliver experience, similar to platform-specific applications. It loads and appears instantly (which looks and feels like a native downloaded app), regardless of the status and quality of the network connection; data exchange over the network significantly speeds up and the UI becomes smoother and more responsive. By using the above-mentioned tools (Service Worker, Push Notifications, etc.), a developer can even achieve offline work support if needed.
Remarkable examples of PWA: Uber, Pinterest, Spotify, Twitter, AliExpress.