Web Services Brief discussion

WebService :
A Web service is a software system designed to support interoperable machine-to-machine interactionover a network.It has an interface described in a machine processable format(WSDL).Other systems intercat with the web service in the manner described by its description using SOAP messages,conveyed using HTTP with an XML serialization in conjunction with other Web related standards.

Why do we use Web Services Instead of DCOM :
Both technologies are from two different technology time zones. COM, DCOM and COM+ are based on the COM architecture (VB6, ATL, C++, MFC), and available in the pre–. NET era, while .NET Remoting is primarily based on .NET Frameworks.
DCOM has drawbacks in the internet connected world because DCOM depends on proprietary binary protocol, which is not supported by all object models and compromises with interoperability across platforms and on the internet, an area where DCOM has severe problems. However, DCOM works well when application systems of similar types exist on the same network. Moreover, DCOM is hard to learn and complicated to deploy, to maintain and to explore.
.NET Remoting supports a set of transport and communication protocols and is more adaptable and easy to deploy to network environments and relatively easy to learn and achieve mastery. DCOM relies on frequent pinging of clients to manage remote object lifetime, where .NET Remoting relies on simple and more efficient leasing mechanisms to maintain object lifetime. DCOM is a closed system, offering little in the way of extensibility, .NET Remoting architecture is much easier to use and extend than DCOM.
Finally, until the arrival of .NET Frameworks, Microsoft's primary protocol for intelligent inter-process communication across systems was Distributed COM. Certainly, DCOM has had some success through pain, and had its day; distributed computing in the Microsoft world will depend on .NET Remoting and Web Services.

Web Service Description :
The mechanics of the message exchange are documented in a Web service description (WSD).The WSD is a machine-processable specification of the Web service's interface, written in WSDL. It defines the message formats, datatypes, transport protocols, and transport serialization formats that should be used between the requester agent and the provider agent. It also specifies one or more network locations at which a provider agent can be invoked, and may provide some information about the message exchange pattern that is expected. In essence, the service description represents an agreement governing the mechanics of interacting with that service.

Semantics :
The semantics of a Web service is the shared expectation about the behavior of the service, in particular in response to messages that are sent to it. In effect, this is the "contract" between the requester entity and the provider entity regarding the purpose and consequences of the interaction. Although this contract represents the overall agreement between the requester entity and the provider entity on how and why their respective agents will interact, it is not necessarily written or explicitly negotiated.
While the service description represents a contract governing the mechanics of interacting with a particular service, the semantics represents a contract governing the meaning and purpose of that interaction

Steps how WebServices utilised:
1.The requester and provider entities become known to each other
2.The requester and provider entities agree on the service description and semantics that will govern the interaction between the requester and provider .
3.The service description and semantics are realized by the requester and provider agents
4.The requester and provider agents exchange messages, thus performing some task on behalf of the requester and provider entities