An API, or Application Programming Interface, is a set of rules and protocols that allows different software applications to communicate with each other. It defines the methods and data formats that developers can use to request and exchange information. APIs enable the integration of different systems, making it easier to build and enhance applications by leveraging existing services. For example, a weather app can use an API to fetch real-time weather data from a remote server. This simplifies development, promotes efficiency, and fosters innovation by allowing developers to build on existing functionalities.