Every application (desktop, web, or mobile) needs to store (and access) some data in order to work properly. How the data is stored depends on the kind of information the application will work with and on the environment in which the application will run. A web application, for instance, can rely mostly on server storage because it runs on the Internet. Most advanced web applications implement an offline strategy and store some data locally on the user machine.
Modern web development offers several tools in order to let users interact with an application even when they're not connected:
The LocalStorage API (http://www.w3.org/TR/webstorage/#the-localstorage-attribute)
The SessionStorage API (http://www.w3.org/TR/webstorage/#the-sessionstorage-attribute)
The ApplicationCache interface (http://www.w3.org/TR/2011/WD-html5-20110525/offline.html)
The IndexedDB API (http://www.w3.org/TR/IndexedDB/)
All the modern mobile browsers let developers handle the online and offline...