SOAP - Simple Object Access Protocol

What is a protocol:
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications. A protocol may have a formal description.
Protocols may include signaling, authentication and error detection and correction capabilities.
A protocol definition defines the syntax, semantics, and synchronization of communication; the specified behaviour is typically independent of how it is to be implemented. A protocol can therefore be implemented as hardware or software or both.
While there is no generally accepted formal definition of "protocol" in computer science,an informal definition, based on the previous, could be "a set of procedures to be followed when communicating". In computer science the word algorithm is a synonym for the word procedure so a protocol is to communications what an algorithm is to mathematics.
SOAP:
It is important for application development to allow Internet communication between programs.
Today's applications communicate using Remote Procedure Calls (RPC) between objects like DCOM and CORBA, but HTTP was not designed for this. RPC represents a compatibility and security problem; firewalls and proxy servers will normally block this kind of traffic.
A better way to communicate between applications is over HTTP, because HTTP is supported by all Internet browsers and servers. SOAP was created to accomplish this.
SOAP provides a way to communicate between applications running on different operating systems, with different technologies and programming languages.

Skeleton SOAP Message

<?xml version="1.0"?>
<soap:Envelope
xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

<soap:Header>
...
</soap:Header>

<soap:Body>
...
  <soap:Fault>
  ...
  </soap:Fault>
</soap:Body>

</soap:Envelope>

A SOAP Example

In the example below, a GetStockPrice request is sent to a server. The request has a StockName parameter, and a Price parameter that will be returned in the response. The namespace for the function is defined in "http://www.example.org/stock".

A SOAP request:

POST /InStock HTTP/1.1 Host: www.example.org Content-Type: application/soap+xml; charset=utf-8 Content-Length: nnn <?xml version="1.0"?> <soap:Envelope xmlns:soap="http://www.w3.org/2001/12/soap-envelope" soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding"> <soap:Body xmlns:m="http://www.example.org/stock">   <m:GetStockPrice>     <m:StockName>IBM</m:StockName>   </m:GetStockPrice> </soap:Body> </soap:Envelope>

The SOAP response:

HTTP/1.1 200 OK Content-Type: application/soap+xml; charset=utf-8 Content-Length: nnn <?xml version="1.0"?> <soap:Envelope xmlns:soap="http://www.w3.org/2001/12/soap-envelope" soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding"> <soap:Body xmlns:m="http://www.example.org/stock">   <m:GetStockPriceResponse>     <m:Price>34.5</m:Price>   </m:GetStockPriceResponse> </soap:Body> </soap:Envelope>