Application programming interface (API) Solution What is an API? An application programming interface (API) is a particular set of rules (\'code\') and specifications that software programs can follow to communicate with each other . It serves as an interface between different software programs and facilitates their interaction, similar to the way the user interface facilitates interaction between humans and computers. How is it used? According to the rules specified in the contract. The whole point of an API is to define how it\'s used. When and where is it used? It\'s used when 2 or more separate systems need to work together to achieve something they can\'t do alone. .