WEB APIs

Machine-based interaction, a common feature of Web 2.0 sites, uses two main approaches to Web APIs, which allow web-based access to data and functions:
REST and SOAP.

1. REST (Representational State Transfer) Web alone to interact, with XML (extensible Markup Language) or JSON payloads

2. SOAP involves POSTing more elaborate XML messages and requests to a server that may contain quite complex, but pre-defined, instructions for the server to follow.

Often servers use proprietary APIs, but standard APIs (for example, for posting to a blog or notifying a blog update) have also come into wide use. Most communications through APIs involve XML or JSON payloads.

For more search : Web Services Description Language (WSDL) (the standard way of publishing a SOAP API) and this list of Web Service specifications.