Think of this as a "chrome-less" web browser. It renders HTML content, without the "chrome" or window decoration of a regular web browser. You build your application to take advantage of this space, and you build navigational/interactive/content elements and application chrome into your HTML and CSS based user interface.
The web view used by PhoneGap is the same web view used by the native operating system. On iOS, this is the Objective-C UIWebView class; on Android, this is android.webkit.WebView. Since there are differences in the web view rendering engines between operating systems, make sure that you account for this in your UI implementation.