A web API

Application programming interface

An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

(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

JSON

JSON , or JavaScript Object Notation, is a lightweight text-based open standard designed for human-readable data interchange. It is derived from the JavaScript scripting language for representing simple data structures and associative arrays, called objects...

 or XML

XML

Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....

. While "web API" is sometimes considered a synonym for web service

Web service

A Web service is a method of communication between two electronic devices over the web.The W3C defines a "Web service" as "a software system designed to support interoperable machine-to-machine interaction over a network". It has an interface described in a machine-processable format...

, the Web 2.0

Web 2.0

The term Web 2.0 is associated with web applications that facilitate participatory information sharing, interoperability, user-centered design, and collaboration on the World Wide Web...

 applications typically have moved away from SOAP

SOAP

SOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks...

-based web services towards more direct REST

Representational State Transfer

Representational state transfer is a style of software architecture for distributed hypermedia systems such as the World Wide Web. The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation...

-style communications.
Web APIs allow the combination of multiple services into new applications known as mashup

Mashup (web application hybrid)

In Web development, a mashup is a Web page or application that uses and combines data, presentation or functionality from two or more sources to create new services...

s.