A Carbon Application (cApp) is a collection of artifacts, which can be deployed on a Carbon instance to easily port your Web services-based solution from one environment to another.
When developing solutions using a WSO2 Carbon platform, there can be many artifact types used in it. For example, Axis2 services, Synapse artifacts (endpoints, Proxy Services, mediators, etc.), Registry Resources, Data Services, etc. Normally these artifacts are developed in a development environment and then moved one-by-one into the staging and production environments. On each and every setup, you have to manually configure these artifacts in order to build up the entire solutions.
WSO2 Web services development tooling environment (Carbon Studio) can be used to develop your individual artifacts and export it as a single cApp artifact. Then you can deploy it on any Carbon server which has cApp deployment capabilities.
For more information about cApp deployment of Registry see following information