Bruce gives a great run-down of what’s involved in creating one of those new-fangled progressive apps that everyone at Google and Opera (and soon, Mozilla) are talking about: a secure connection, a service worker, and a manifest file.
Crucially, in browsers that don’t support it, you have a normal website. It’s perfect progressive enhancement.
Funnily enough, this here website—adactio.com—is technically a progressive app now.
At their simplest, Progressive Web Apps are application-like things hosted on your web server. If you’re as old as me, you might call them “web sites”