One critical feature of applications is that they usually need to store the user's data persistently. Until now, we've been using localStorage for our application, but it would be even better if we could use the device's actual filesystem.

This is where the File API comes in. It's actually not specific to Cordova; there are several browsers that support the File API (to varying degrees). The specification itself is defined by the W3C (http://www.w3.org/TR/FileAPI/ and http://www.w3.org/TR/file-system-api/).

The File API gives you the ability to access your app's sandboxed filesystem. That means you can create folders and files, move them around, copy them, edit them, and delete them, just like you can with native code.

There's only one downside: the API itself is largely asynchronous, which usually means venturing into "callback hell"; not a very fun place to be. It can be incredibly difficult to debug a callback chain or prevent such a chain from devolving into spaghetti...