What happens to versions when we publish a page through workflow in CQ5?

CQ5 Creates a version by its own of the page when you publish page by workflow. It is a new version of your page.

Which browser does phonegap use ?

PhoneGap uses a webview based off the default OS browser, in iOS (Safari), Android 4.1 above (Chrome since chrome was made the default browser), older Android devices, it used Android Browser which was based off webkit.