This document discusses SOAP (Simple Object Access Protocol), the standard messaging protocol used by web services. It describes SOAP as a lightweight protocol that allows applications to exchange messages and data between disparate systems. The key topics covered include SOAP's use as an inter-application communication protocol, the structure of SOAP messages containing an envelope with optional header and mandatory body elements, and how SOAP messages are transported over HTTP. The document also discusses SOAP headers, bodies, faults, and intermediaries in the SOAP communication model.