Problems in traditional web Applications

1. Many round trips to the server.
2. Rendering of webpage is slower.
3. Increases the consumption of server resources.
4. Response time of application is very slower.
5. Rendering of data is static.
6. Data is updated of an application only when the user clicks on the browser refresh button.

So the preceding is some of the basic problems related to the traditional web application so to overcome these problems Ajax is the best alternative. So let us start with the basics.
What is Ajax ?
Ajax stands for Asynchronous JavaScript and XML; in other words Ajax is the combination of various technologies such as a JavaScript, CSS, XHTML, and DOM etc.
AJAX allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes. This means that it is possible to update parts of a web page, without reloading the entire page.
We can also define Ajax is a combination of client side technologies that provides asynchronous communication between the user interface and the web server so that partial page rendering occurs instead of complete page post back.
Ajax is platform-independent; in other words AJAX is a cross-platform technology that can be used on any Operating System since it is based on XML & JavaScript. It also supports open source implementation of other technology. It partially renders the page to the server instead of complete page post back. We use AJAX for developing faster better and more interactive web applications. AJAX uses a HTTP request between web server & browser.
Using Ajax technologies we can observe in Google Maps, Gmail, YouTube, and Facebook tabs.
I hope you have understood the basic concepts of Ajax. Now let us see the following diagram showing how Ajax works:

In the above diagram it's clear how Ajax works, that first depending on the client requests the browser creates the XMLHttpRequest and sends it to the server and there after that the server processes the HttpRequest given by the browser, processes it and then sends the response back to the user and at the end the browser processes the response given by the server using JavaScript and update the requested content of the particular page instead of the entire page contents.
AJAX is Based on Internet Standards
AJAX is based on internet standards, and uses a combination of a XMLHttpRequest object to exchange data asynchronously with a server, JavaScript and DOM to interact with the information and CSS is used to style the data and XML is often used as the format for transferring data.
Let us know about the basic information each technology uses in Ajax.
1. XHTML and CSS
2. Document Object Model (DOM)
The Document Object Model is a platform and language independent standard object model for representing HTML or XML.
3. XML
XML is typically used as the format for transferring data between the server and the client. Using XML we can represent any applicable data object structure we might wish to represent.
4. XMLHttpRequest (XHR) and JavaScript
The XMLHttpRequest is the core of the Ajax model; without it the model would not exist. The XMLHttpRequest JavaScript object is the enabling technology which is used to exchange data asynchronously with the web server. In short, XMLHttpRequest lets us use JavaScript to make a request to the server and process the response without blocking the user. Naturally, as we are using this JavaScript object, the providing technology is JavaScript and hence some knowledge of JavaScript is required to get Ajax applications to function. 

•    In Ajax Client and Server Communication done with help of HttpRequest.
•   AJAX applications are browser and platform independent.

1. Using ScriptManager
   
   When we use any Ajax control then there is a requirement to use the ScriptManager to handle the Scripting on the client side; without the ScriptManager Ajax controls are not run. So it's a requirement to use the ScriptManager.
    
2. Using UpDatePanel
   
   Update panel is one of the most commonly used Ajax controls which is responsible for updating the particular requested content of the page instead of the entire page which is not requested. The ASP.Net controls are put under the update panel to make the benefit of this control. The ASP.Net controls which are kept under the update panel will be updated when the user clicks on a particular ASP.Net Control which are used in an application.
   You can use multiple update panels on a single web page.
    
3. Using Timer
   
   The Timer is also one of the important controls; by using it we can update the particular content of the page automatically without clicking the browser refresh button. The Timer control is used along with the Update Panel so the Contents put under the update panel are automatically updated according the timing set under the timer_click event.
    
4. Using Update Progress
   
   This control is used to notify the user to wait until the requests are processed on the server. Update progress control is used when the user clicks on any tab or control of an application. At that time the progress bar is shown which is the interval between the times taken by the server to process the client request.
    
5. Using ScriptManagerProxy
   
   When you need to reference a service from your content page and yet the ScriptManager resides on the Master Page use a ScriptManagerProxy. The ScriptManagerProxy works by detecting the main ScriptManager on your page at runtime and hooking itself to that ScriptManager, making sure that any references given to it are also given to the real ScriptManager.
    
6. Using Pointer
   
   This Ajax Control used to specify the style of the mouse pointer such as arrow, thumb, and progress bar and much more. The above is the basic introduction about the Ajax Extension controls which are available by default in a Microsoft Visual Studio Framework.

Summary
In this first part of article I have explained the basics of Ajax. In my next article of the series, Using Ajax Extension :Part-2, I will explain how to use Ajax Extension controls in an ASP.Net C# web application.