SimpleObjectAccessProtocol (SOAP) related to a family of standards designed for the implementation of WebServices. RestInSoap has links to alternatives to using Soap.
DonBox, the father of Soap who now works for MicrosoftCorporation, wrote an article called "Young person's guide .." that discussed the rationale for having Soap. Link is http://msdn.microsoft.com/msdnmag/issues/0300/soap/default.aspx.
People interested in technical background should check out http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwebsrv/html/xmloverchap2.asp for introductory ExtensibleMarkupLanguage, and perhaps visit IwannaLearnXml as well.
For handson person, if you are working / learning in a Microsoft environment, then SoapToolkit is the door to learning all that WebServices fuss, see article at http://msdn.microsoft.com/msdnmag/issues/0800/webservice/default.aspx
Develop Mentor has a Soap discussion list accessible at http://discuss.develop.com/soap.html
If we go beyond basics, there are demands for SOAP to carry binary (opaque) data, see http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwebsrv/html/infoset_whitepaper.asp. Another important requirement is to have security enhancements. Additional information can be found at http://www.idevnews.com/TipsTricks.asp?ID=124
Feb05 W3C Primer for Soap 1.2 with XmlPerformance enhancements at http://www.w3.org/2000/xp/Group/2/06/LC/soap12-part0.html