Introduction
Web Service ?? Shouldn't that be Windows Service (Win Services) ? Hmmm... Nopes its Web Services !! With the introduction of the new .NET platform there has been a creation of "Web Services". 
So one more type of service to learn, understand --- yep but one thing is that its pretty easy to learn and master. Note this example is specific to .NET SDK beta1 and won't work on any other version of the SDK.

So what is a Web Service ?
The simple answer to that would be a,  "Application running as a service delivered over the Web". If I was to break up this statement there are few points which get highlighted. 

1) A Web Service is a Application.
2) To be more precise its a Application running as a Service (like Windows Services in Windows 2000) . So one thing is cleared that once deployed a Web Service will run in the Background for 24x7 (24 hours a day, 7 days a week).
3) "Delivered over the Web" - This statement says a lot of things about Web Services. Firstly Web Services are URL addressable i.e. You can call (invoke) it from your browser by typing the path to the Service e.g. "http://localhost/myservice.asmx" . 
If you think a bit more deeper over the point "Delivered over the Web" and if you know a bit about the Protocols used over the Web you can easily make out that Web Services have to use either of the 2 protocols widely used on the Internet "HTTP or FTP". Since FTP is to transfer files to using it is out of question , hence no prizes for guessing that Web Services use HTTP protocol for communication with its clients. That means that you do not need any special software to host and run your Web Services, your normal Web Server will be able to serve your Web Services...(there you save your precious time and Brain in learning that a new Server first that runs your application !!). As stated above Web Services uses HTTP for communication with its clients, this is a very big advantage over other remote services like DCOM , RMI etc which have their own Object -Specific Protocols. 

The Web Service interface can be defined strictly in terms of Messages a Web Service Accepts and Generates. This lead to a conclusion that Web Services talk with its clients with Messages. Due to XML being the Internets Favorite of describing Data, Microsoft has used XML extensively in Web Services. The Messages transferred to/fro from the Web Service are in XML format. Another standard used to access a Object remotely is SOAP (Simple Object Access Protocol). Web Services use SOAP encoded XML messages to communicate. Does that mean you have to learn XML and SOAP ?? Not exactly, that's the beauty of Web Services which leads to some people calling it as having "Black - Box" functionality. What they mean is that you can code and use Web Service with out knowing exactly how the Web Service is implemented i.e. You will never need to worry about how the messages get coded into SOAP and XML and how they are transferred to it clients. 
A very important point that many of you might have already understood would be that the Clients (consumers) of the Web Service could be from any platform, Object model and Programming Language. As long as they can understand and generate SOAP encoded XML messages , You guessed that, didn't you? This solves a big problem of application interoperability being experienced by today's programmers. 

Future of Web Service?, will all my applications get converted to Web Services? Impact of Web Services? 
Hey, hey hold on a second... I am just 21 and still getting a gist of how the Programming World Functions!! It would be better if you ask those questions to seasoned developers, having a experience of 10-20 years out there who would let you and Me know, of this technology's future? As for me, I am all excited and can find many applications for it in the present Web scenario. One such field I think where Web Services could be a major boon would be in the field of "Content Providers" , as I call them. If you are a regular browser of many "Consumer Oriented Portals" which provide daily updates of News, Sports, Stocks, Games, Weather, E-mail, .... and not to forget Horoscopes (That's what I look up every day to count my chances of emerging as a good programmer :) )!!!  The point I want to make here is that you will find that many of these "Consumer Oriented Portals" actually do not create original content in all the sections. They subscribe to some other Portals which provide specialized information on that section, that's the reason why if you surf many such portals you will find that the content of some of their columns like Sports, Weather etc are the same.  Such portals which offer specialized information are called as "Content Providers", since they provide specialized content to other Portals as well as to general surfers directly. 
I have tried here to outline a hypothetical Web Service which such Content Providers could use. Guess what I will be delivering as Content through my Web Service? ........  Ahmmm Horoscopes ........ .
Yep this is what my sample service would deliver, Daily Horoscopes. Since I am not a astrologer I will not provide accurate readings nor will I be updating it daily.... :) but still I will try to give it my best shot !

Design Considerations
A) Server Side ("Content Provider Side")
   The horoscopes will be updated Daily by the staff. The daily horoscopes will be stored in a XML file called dailyhoro.xml. 
B) Clients ("Content Consumers")          
   The clients can view their Horoscopes through Web Pages or through Applications. They will provide the Zodiac sign and the Web Service will process their request and send them their daily readings.

Xml Database File Format

Code

Time to kick up some real code. First we will see the code for the Web Service. This Service is pretty simple, it takes a string containing the "Zodiac Sign" as a input parameter. Then it looks up its database for the prediction for the "Zodiac Sign" the client entered. If a match if found then it returns the user a string containing the Date for which the horoscope is along with the prediction for the supplied sign. Just incase you did not know there 12 zodiac signs namely - "Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Capricorn, Sagittarius". What is your Zodiac Sign ??? Mine is encoded in this example if you look carefully !

1) wshoro.asmx :- The Horoscope Web Service

<%@ Webservice Language="C#" class="DailyHoro" %>
using System ;
using System.Web.Services ;
using System.Xml ;
using System.Data;
using System.IO ;

//Define a Namespace for our Web Service
//Define a Description for our Web Service
[WebService(Namespace="http://www.mastercsharp.com/WebService"
  Description="Web Service to deliver daily horoscopes")]
public class DailyHoro: WebService
{


  //All the methods you want to expose to the Web Service should
  //also be public methods.
  //To make the method available to the Clients you have to mark the
  //method with the attribute "[WebMethod]"
  //only the marked methods will be available to the clients
  //This method has one Input parameter as a String containing the Zodiac
  //sign name of the Sign for which the client wants the Horoscope.
  //In return we send the client the current date and the prediction.
  [WebMethod(Description="Returns the prediction for the given Zodiac sign")]
  public string GetHoro(string zodiac)
  {

    //We give a call to our private method which does all the data collection
    //and sends the Current Date and prediction of the sign requested
    //We return the data to the client
    return getData( zodiac.ToLower() ) ;

  }


  //A Private method which takes the "Zodiac" sign as input and returns
  //a string containing the current date and the Prediction for that sign.
  //Since I am using XML as the Database, this method first reads the Data
  //Then it searches for the specified sign in the database
  private string getData(string zodiac)
  {
    try{
      //Declare a FileStream
      FileStream fin ;
      //Set the Various parameters on the File stream.
      //It is also to declaring "FileShare.ReadWrite"
      //which will allow other users too to read this file
      //Also the System Admins will have access to update the File.  
      fin= new FileStream(Server.MapPath("db/dailyhoro.xml"), FileMode.Open,
	FileAccess.Read,FileShare.ReadWrite )  ;
      //Create a DataSet object to contain our Database
      DataSet ds = new DataSet() ;
      //Read the Xml Schema and Data from the stream
      ds.ReadXml(new StreamReader(fin)) ;
      //close the stream
      fin.Close() ;
      //loop to check for the Zodiac sign provided
      //We check up to 12, ??? any answers why ??
      //Didn't get it may be it too much coffee ..
      //There are only 12 Zodiac signs -Remember!!!
      for(int i=0;i<12 ; i++)
      {
	//Check if the Zodiac sign provided by the user Equals
        //the "name" field in our Table "zodiac" 
	if(zodiac.Equals( ds.Tables[0].Rows[i]["name"]) )
        {
          //If a match if found then return the Date and
          //Prediction 
	 string pred =ds.Tables[0].Rows[i]["pred"].ToString() ;
          string date = ds.Tables[0].Rows[i]["today"].ToString() ;
	 return date+"@"+pred ;
        }
      }
      //If no match found then return a error statement 
      return "Wrong Parameter" ;
    }
    catch(Exception ed)
    {
      //catch any error and inform the user a error has occurred
      //in the Web Service
      //You will not do this in real implementation
      //Keep the user guessing or send him some understandable message
      //like service down etc..
      return "Read Error!! :"+ed.ToString() ;
    }
  }
}

That's It !!! Saw how easy it is to write a Web Service. You don't even have to think how the Service will listen for incoming clients or the communication will take place between the Service and its Clients. Save the file as "wshoro.asmx". Web Services are stored with the extension ".asmx" .

Web Service Deployment
To deploy a Web Service is as simple as deploying any other ASP.NET Web Application. In our example create a Virtual Directory on your computer if you are using a "localhost" Server, or simply upload your files to any server supporting ASP.NET. Remember to place the "dailyhoro.xml" file in a directory called "db" in the virtual directory. Now your web Service is deployed, to call it just type the URL of the Web Service in to your browser. The page that you see is auto generated by the .NET runtime. It gives some information about the Web Service.

Consumers
Since your web service is ready you can now proceed with writing clients to consume your service.

1) Proxy Class
In order for the client to use the Web Service they should have knowledge of how the Web Service is implemented and how to call it etc. But as I said before that you can talk to Web Services with out knowing how they are implemented? You are right but theoretically the client cannot know automatically about the Web Service. Hence all Web Services automatically generate a XML document with WSDL(Web Service Description Language) grammar to describe itself. 

In our example say if you have installed the Web Service in a Virtual Directory called "horoservice" then you can get the WSDL of the service by calling the following URL in your browser "http://localhost/horoservice/wshoro.asmx?WSDL". This will generate a XML file and display it in your browser.

The .NET SDK has a tool called "WSDL". This tool takes the WSDL of a Web Service as a input and uses Reflection to generate a Proxy Class that is similar to the Web Service. To create a proxy class for our Horoscope Example use
WSDL http://localhost/horoservice/wshoro.asmx?WSDL /n:horo_service

Where "http://localhost/horoservice/wshoro.asmx?WSDL" -- is the full URL to the WSDL.
Remember the WSDL of the Web Service and not the Web Service.
So you can also save the WSDL of the Web Service to your Computer and use it for creating a Proxy.
"/n:horo_service" -- is to specify the Namespace to create for the Proxy Class.

This will generate a file "DailyHoro.cs" in the directory you called WSDL. This file contains the source code which defines the Web Service in C#.
Now compile this Source code to generate a Library that we can use in our clients. The library generation command will be.
csc /target:library  DailyHoro.cs
The above code will generate a Dll "DailyHoro.dll" . With this you have successfully created your Proxy Library and now you can easily write up clients to use your Web Service.

2) ClientHoro.aspx:- The Web Page Client
Are you awake ??? Grab a cup of coffee since we are on a part which many of you might be knowing well by now. We are going to write a ASP.NET Web page which will consume the Web Service we have just written.

Deploy the Web Application
To deploy the above Web Page is similar to deploying any ASP.NET Page. Copy the File ClientHoro.aspx to your Virtual Directory. Also remember to copy the Proxy Dll to your "Bin" directory located in the Virtual Directory. If a "bin" directory does not exist then create it in your Virtual Directory. 
Wasn't that easy to consume the Web Service. Also since the input is returned in the form of a string you can format that data to suit to your needs.

2) HoroClient.cs:- The Horoscope Web Service Consuming Console Client

Compile this code and you will get the Executable "HoroClient.exe" which consumes our Web Service. Run the file to use the service.

Conclusion
If you have gone through the article you will find that Writing and Consuming Web Services on the .NET Platform is very easy. Also in case of our "Content Providers" example Web Service can be very useful since the consumers take the responsibility of how to format the Code. Also one useful point for the consumers is that they have to write the code only once. In our examples the Consumer Portals have to just write the code once, Their content gets updated every time the content gets updated on the Web Service.
Have you got some extra coffee send it over to me ... my coffee is over and I am dam sleepy (so if there are any errors in the code do let me know :) )!!!!!!

Points to Remember
1) Web Services are Applications Delivered over the Web.
2) Web Services use HTTP protocol to communicate with its client.
3) Web Services use SOAP encoded XML messages to communicate with its clients.
4) Web Service clients can be anything from Web Pages to Applications to Devices only criteria being the client can understand the and communicate over HTTP in SOAP encoded XML messages.
5) WSDL is the description of a Web Service generated automatically for all Web Services.
6) If you are using the .NET platform to create a Client then use "WSDL.exe" to create a Proxy class from the WSDL of the Web Service.
7) Once you have the Proxy class code, compile it into a Library.
8) Use the Proxy class Library in your Clients to call methods upon a Web Service.
9) Hey if someone really implements this service for the public then please do let me know, since I would have but I don't have the resources yet ....