Introduction to Web Services

Overview
The label "web services," as broadly applied, has two levels of meaning—one specific and one conceptual:

Specifically, web services are a stack of emerging standards that describe a service-oriented, component-based application architecture.

Conceptually, web services represent a model in which discrete tasks within e-business processes are distributed widely throughout a value net.

Web services are Loosely coupled, reusable software components that semantically encapsulate discrete functionality and are distributed and programmatically accessible over standard Internet protocols.

Going into an examination of the definition

First, web services are reusable software components. Web services continue the long ascension of object-oriented design in software development. Rather than requiring programmers to write one start-to-finish set of instructions after another, the component-based model allows developers to reuse the building blocks of code created by others to assemble and extend them in new ways.
Second, these software components are loosely coupled. Traditional application design depends upon a tight interconnection of all subsidiary elements. The complexity of these connections requires that developers thoroughly understand and have control over both ends of the connection; moreover, once established, it is exceedingly difficult to extract one element and replace it with another. Loosely coupled systems, on the other hand, require a much simpler level of coordination and allow for more flexible reconfiguration.
Third, web services semantically encapsulate discrete functionality. A web service is a self-contained "applet" that performs a single task. The component describes its own inputs and outputs in a way that other software can determine what it does, how to invoke its functionality, and what result to expect in return.
Fourth, web services can be accessed programmatically. Unlike web sites and desktop applications, web services are not designed for direct human interaction, and they do not have a graphical user interface. Rather, web services operate at the code level; they are called by and exchange data with other software. Web services certainly will be incorporated into software designed for human interaction, however.
Finally, web services are distributed over the Internet. Web services make use of existing, ubiquitous transport protocols like HTTP. Web services leverage existing infrastructure and can comply with current corporate firewall policies.

Core Layers of the Web Services Stack

Common Internet Protocols - Although not specifically tied to any transport protocol, web services build on ubiquitous Internet connectivity and infrastructure to ensure nearly universal reach and support. In particular, web services take advantage of HTTP, the same connection protocol used by web servers and browsers.
Extensible Markup Language (XML) - XML is a widely accepted format for exchanging data and its corresponding semantics. It is a fundamental building block for nearly every other layer in the web services stack.
Simple Object Access Protocol (SOAP) - SOAP is a protocol for messaging and RPC-style communication between applications. It is based on XML and uses common Internet transport protocols like HTTP to carry its data. SOAP has been submitted to the World Wide Web Consortium (W3C) standards body and will emerge later this year as "XML Protocol (XP)".