Phonegap allows you to embed a standard HTML5 web site/page/app into a native "shell", for running on iPhone/Android and submitting to the app-stores. It also includes some advanced APIs for device access. The app you interact with is actually a web-view control.

Mouin is a set of vendor-specific CSS extensions you can add to your regular, online HTML content, and the Mouin client app can read these and create native iPhone/Android/BB/J2ME OS UI controls. The UX will be more native, while content is distributed as a regular web page.

Sorry if the explanation is complex, mobile is a really complex space right now. Actually, PhoneGap and Mouin are similar in allowing web devs and designers to create native mobile apps. How they allow it and the resulting apps are radically different in every aspect.