An Application Programming Interface (API) is a set of commands that allows an application to interact with an external service, saving time and resources by leveraging the underlying services of another application. APIs are used in many applications, including popular ones like Skyscanner for accessing flight schedules and prices. A microservice architectural pattern is a modular approach to developing applications, where loosely coupled services work together to create a complete application, with each service having a specialized role. Microservices can be independently deployed and serve specific purposes, such as customer information or payment processing, making them scalable and flexible alternatives to monolithic architectures.