Sencha Touch is an HTML5 mobile framework. It will help you to develop a web application with HTML, JavaScript and CSS. There are more similar frameworks, like jQuery Mobile or Lungojs.
On the other hand, PhoneGap is a platform that allows you to use mobile devices features, such as GPS, camera, etc., through a JavaScript interface.
In conclusion, to create an app using these technologies, you would implement the design and the logic in HTML5 (with Sencha Touch if you wish), would use PhoneGap to make use of the device features and wrap the app as a native application.