Aِpplication Programming Interface (API) is a set of subroutine definitions, protocols, and tools for building application software. It is a set of clearly defined methods of communication between various software components. A good API makes it easier to develop a computer program by providing all the building blocks, which are then put together by the programmer. An API may be for a web-based system, operating system, database system, computer hardware or software
In order for you, or other applications to interface with recurly, you'll need to use an API key. Public key is the one that you can distribute to others, to allow them to get some subset of information about you from the API. Private key is for your use only, and provides access to all of your data.