In technology definitions are important. We talk in a language that's mostly made up by our own industry.
There there's "web app". A lot of people (including myself) will refer to many web sites as apps. But why? I don't know of a good definition ("good" in that there's no hand-waving involved).
So here's my definition and distinction between web site and web app.
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A URL that you visit. Classic examples include a blog or a wiki. Where the content is waiting for you at the end of the URL.
JS Bin is a web site by this definition. You visit the URL and create content. Someone will share a bin, and you visiting this content. You go to the site.
A URL that you take with you. This is locally installed and data is pushed to you. Classic examples of this are email clients, news readers, task orientated applications. The app goes with you.
Importantly: you expect it to be available with or without an internet connection.
Isn't this just a web site with offline support?
I'm still scoffing at "just". Sure, if you want to dumb it down as such. But equally isn't an app on your screen, like Chrome or TomTom or PhotoShop, just some compiled code? Yes, it is, but it's actually a lot more than that.
The common language word we refer to compiled programs are: apps.
It's not that it's harder to build web apps, it's that the experience with these types of web sites are fundamentally different to a web site like Wikipedia for instance.
The future of web browsers is coming, and they're giving us more and more support for offline technology, through storage (for data caching) to control over the network (through service workers).
Now, next time you're asked to build a web app, at least you can have a feature set that you're working towards.