Application-Layer protocol defines how an application's processes, running on different
end systems, pass messages to each other. In particular, an application-layer protocol
defines: the types of messages exchanged, for example, request messages and response messages, the syntax of the various message types, such as fields in the message and how
the fields are delineated, the semantics of the fields, that is, the meaning of the information
in the fields, and rules for determining when and how a process sends messages and
responds to messages.

It is important to distinguish between network applications and applications-layer
protocols. An application-layer protocol is only one piece of a network application. The
Web application consists of many components, including a standard for document
formats, Web browsers, Web servers, and an application-layer protocol. The Web's application-layer protocol, HTTP, defines the format and sequence of the messages tha
t are passed between browser and Web server.