A lot of them are, but they are differentiated by the run-of-the-mill database front-end CRUD app by their concept.

E.g, is Twitter a CRUD app? At the programming level maybe, but the core idea behind the service is quite different.

And there are the more interesting non CRUD apps. Google Docs. Google Maps. Photoshop.com. Multiplayer online Games. Online Editors. Online Validators. Real-time collaboration apps. Etc etc.

Not necessarily. A CRUD app is an app whose primary capability is allowing the user to interact with a relational database for storing and retrieving records.

Google Search is one obvious example of a web app that has absolutely nothing to do with CRUD. It uses a search algorithm to search a massive priority-ranked data structure that represents an index of many millions of popular web pages.