Because distributed applications are becoming so popular, SOAP, which lets such applications communicate, is becoming popular, too. Plenty of SOAP resources can be found on the Internet; here's a starter list, including W3C's documentation on it:
http://www.w3.org/TR/SOAP/— The W3C SOAP 1.1 documentation. This is a W3C Note.
http://www.w3.org/TR/SOAP-attachments— The W3C SOAP Messages with Attachments documentation. This is also a W3C Note.
http://www.zvon.org/xxl/soapReference/Output/index.html— A SOAP 1.1 reference.
http://www.xml.com/pub/a/2001/04/04/soap.html— A history of SOAP.
http://www.develop.com/soap/soapfaq.htm— The SOAP Frequently Asked Questions (FAQ).
http://xml.apache.org/soap/faq/index.html— The Apache project's SOAP FAQ.