API is an acronym for Application Programming Interface. It is clear from the name that while performing API testing we will be doing testing at backend which do not offer GUI rather works at back-end.
API is set of rules or standards for accessing a web based service. A software company releases its API so that others can design products that are powered by their services.
API’s are software to software interface not the user interface. With API’s web applications talk to each other without any user intervention or knowledge.
APIs and Web services are completely invisible to Web site surfers and software users. Their job is to run silently in the background, providing a way for applications to work with each other to get the user the information or functionality he needs.