High Quality Content by WIKIPEDIA articles! A web API (Application Programming Interface) is typically a defined set of HTTP request messages along with a definition of the structure of response messages, typically expressed in JSON or XML. While "web API" is sometimes considered a synonym for web service, the Web 2.0 applications typically have moved away from SOAP-based web services towards more direct REST-style communications. Web APIs allow the combination of multiple services into new applications known as mashups.