APIs, or Application Programming Interfaces, are tools that allow different software systems to communicate with each other. They act as bridges between applications, enabling them to share data and functionalities. APIs simplify complex functions and make them accessible through easy-to-use interfaces, allowing developers to interact with them using just a few lines of code. There are different types of APIs, including internal APIs used for a company’s internal operations, public APIs released for developers to build on, and vendor APIs like Stripe, which provide products in the form of APIs.

APIs use various protocols and technologies, with SOAP, REST, and gRPC being the major implementations. SOAP, based on XML, is considered legacy, while REST, based on URLs and representing resources, is widely used. gRPC, introduced by Google, is gaining popularity for distributed systems optimizing for scale and low latency. To make API requests, developers need to understand endpoints (URLs), methods (GET, POST, PUT, DELETE), request bodies (data), and headers (metadata). They can interact with APIs through browsers, command-line tools like cURL, client libraries in programming languages, or specialized tools like Postman.

Advanced topics in API usage include authentication, error handling, and caching. Authentication ensures secure access to APIs, with methods like Basic and Bearer authentication. Error handling involves interpreting status codes in API responses, such as 400 for a bad request or 403 for forbidden access.