For any SharePoint 2013 apps to deploy and be of use to anyone, they must be first packaged into .app packages, which in turn are Open Packaging Convention (OPC) packages, and therefore Zip files in disguise. If you rename them with a .zip extension you can open and extract them much like you would any other Zip package. The OPC packaging format is fully documented and standardized. You can read more about it and working with packages at: http://msdn.microsoft.com/en-us/magazine/cc163372.aspx.
A SharePoint app package has four main parts:
For those developers familiar with full-trust solution .wsp files in SharePoint 2010, this concept of a single file containing multiple subparts might seem familiar (excluding the database package portion). In SharePoint 2010, WSP solutions also included a manifest, files and assets, and code packages (DLLs). However, in SharePoint 2013 the format and deployment of these assets varies greatly.
The manifest in an app package contains important information such as the app’s name and version.
The SharePoint solution package contains XML-based declarations for SharePoint components such as lists and content types that should be deployed as part of the app.
The Web deploy package is the code portion of your app. It is the package that will be deployed to Azure as part of an Autohosted app.
Finally, the database package contains the SQL Azure deployment package ...