An API, or Application Programming Interface, is a set of functions or code that enables applications to access data and interact with external software components, operating systems, or microservices. In simpler terms, an API works by sending a user request to a system and then returning the system’s response back to the user. For example, when you click “add to cart,” an API tells the website that you added an item to your cart, and the website places the item in your cart, and your cart is updated. The computer communicates in its local language, called programming language, and the API tells it what to do.

APIs serve as a point of interaction between computers and programs. This communication is done using a programming language called JSON, which can also be used to perform defined actions like updating or deleting data. Four basic request methods can be made with API: GET, PUT, POST, and DELETE. JSON is used to represent data on a server, which is easy for humans to read and machines/applications to understand.

APIs can be classified as private, partner, or public based on their delivery solutions. Private APIs are designed for improving solutions and services within an organization. Partner APIs are openly promoted but shared with partners who have agreed to a deal with the distributor. Public APIs, also known as developer-facing or external, are available to any third-party developers.

APIs can also be defined by the systems for which they are designed. Database APIs enable communication between an application and a database management system. Operating system APIs define how applications use the resources and services of operating systems. Remote APIs define rules of interaction for applications running on different machines. Web APIs, which are the most common, provide machine-readable data and transfer between web-based systems that represent client-server architecture.

There are different API protocols or conventions that are used to standardize data exchange between web services. Remote Procedure Call (RPC), Service Object Access Protocol (SOAP), Representational State Transfer (REST), and GRPC are some examples. REST, unlike SOAP, is a software architectural style that has six requirements for building applications that work over HTTP, typically web services. GRPC is an open-source universal API framework that is also defined under RPC.

To ensure good documentation for APIs, it should have quick start guides, authentication information, explanations for each app call (request), examples of each request and response with response description, error messages, and code samples for popular programming languages like Python, Java, JavaScript, or PHP.

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