What are SharePoint App Shapes ?
SharePoint 2013 allows for multiple ways to interact with and access apps. The key method used to determine how your app will be interacted with is to define the one or more app shapes. App shapes define the experience for interacting with and using an app. There are currently three app shapes available:
Immersive (or full-page app): This shape provides a fully immersive experience by using the entire page. While this gives you complete control over the app experience, it is important to make sure that your app properly links back to the SharePoint site, so the user experience feels integrated and not lost. To make this simple, we provide a chrome control that not only lets your app automatically point a user back to their SharePoint environment, but it also provides your app with the current SharePoint cascading style sheet (CSS). So when SharePoint changes its CSS, say, through a theming change, your app will change its look and feel as well.
App part: An app part is an IFrame hosted in a Web Part that is represented by the ClientWebPart class. The IFrame can take parameters which can change the user experience of the app part. You can think of these app experiences as similar to standard Web Parts, the difference being they do not interact in the same way legacy Web Parts do.
Extension app: Extension apps are just custom actions that can direct to a page in your app. This could be a page hosted on SharePoint, or, in the case of a cloud-hosted app, a remotely hosted webpage. You can think of extension apps as entrance points to your app. They don’t provide a user experience in themselves.