Progressive Web Apps don't have a strict definition. The best way to think of them is as a higher level of web application which contains a particular set of powerful features.
Possible features include
- Offline ability
- Caching (for speed reasons!)
- Installability
- Device-agnosticity
- Push notifications
- Bluetooth access
- Media Controls
- Device Stats
Hold up. Aren't we talking about a website? Why does it say offline ability and installability?
The true power of a PWA is that it can be installed to a user's device, just like a traditional mobile app. It behaves like a mobile app as well. Even better, it can be installed to desktop devices as well!
Try it out right now on mobile.twitter.com!
For the price of just one website, you can produce something that works on web, mobile and desktop. Hard to pass up.