What makes a web-app an app? Id argue its not the functionality theres a lot of functionality on the web that we wouldnt explicitly consider a web-app. Id argue its the account, the infrastructure, the multi-page UI, the navigation between elements, the billing, the overview pages, etc. Thats the stuff that makes a web-app an app.
Now, what if we removed all of that? What if a web-application looked more like a web-document? What if the whole application was a single document, a single URL? What if the interface was one page? No account, no preferences, no settings, no navigation in the traditional sense. What if you could email it around just as you do a photo or a file attachment?