From the Computing dictionary at: http://computing-dictionary.thefreedictionary.com

(Simple Object Access Protocol) A message-based protocol based on XML for accessing services on the Web. Initiated by Microsoft, IBM and others, it employs XML syntax to send text commands across the Internet using HTTP. SOAP is similar in purpose to the DCOM and CORBA distributed object systems, but is lighter weight and less programming intensive. Because of its simple exchange mechanism, SOAP can also be used to implement a messaging system. SOAP is supported in COM, DCOM, Internet Explorer and Microsoft's Java implementation.

For an implementation example, please check out Using PeopleSoft SOAP Classes