SOAP is one of the primary protocols in the Web Services category of distributed computing.

Along with UDDI and WSDL, SOAP is often considered the third primary pillar of the Web Services specifications. Where WSDL defines the service interface, and UDDI acts as the directory of interfaces, SOAP acts as the description language for remote invocations, message serialization & de-serialization as well as identifies bindings to underlying transport protocols.

The following is an extract from the SOAP specification:

"SOAP is a lightweight protocol for exchange of information in a decentralized, distributed environment. It is an XML based protocol that consists of three parts: an envelope that defines a framework for describing what is in a message and how to process it, a set of encoding rules for expressing instances of application-defined datatypes, and a convention for representing remote procedure calls and responses. SOAP can potentially be used in combination with a variety of other protocols; however, the only bindings defined in this document describe how to use SOAP in combination with HTTP and HTTP Extension Framework."

Most current implementations of SOAP are SOAP version 1.1. However, SOAP 1.2 is identified as the successor, but may not gain acceptance until either the WS Basic Profile is versioned or a new profile is identified (incorporating GXA features).