ASP.NET 
ASP.NET, the next version of ASP, is a programming framework used to create enterprise-class Web Applications. These applications are accessible on a global basis leading to effecient information managment. The advantages ASP.NET offers is more than just the next version of ASP.
Why ASP.NET?
Since 1995, Microsoft has been constantly working to shift it's focus from Windows-based platforms to the Internet. As a result, Microsoft introduced ASP (Active Server Pages) in November 1996. ASP offered the efficiency of ISAPI applications along with a new level of simplicity that made it easy to understand and use. However, ASP script was an interpreted script and consisted unstructured code and was difficult to debug and maintain. As the web consists of many different technologies, software integration for Web development was complicated and required to understand many different technologies. Also, as applications grew bigger in size and became more complex, the number of lines of source code in ASP applications increased dramatically and was hard to maintain. Therefore, an architecture was needed that would allow development of Web applications in a structured and consistent way.
The .NET Framework was introduced with a vision to create globally distributed software with Internet functionality and interoperability. The .NET Framework consists of many class libraries, includes multiple language support and a common execution platform. It's a very flexible foundation on which many different types of top class applications can be developed that do different things. Developing Internet applications with the .NET Framework is very easy. ASP.NET is built into this framework, we can create ASP.NET applications using any of the built-in languages.
Unlike ASP, ASP.NET uses the Common Language Runtime (CLR) provided by the .NET Framework. This CLR manages execution of the code we write. ASP.NET code is a compiled CLR code instead of  interpreted code (ASP). CLR also allows objects written in different languages to interact with each other. The CLR makes developement of Web applications simple.
Advantages Using ASP.NET

• ASP.NET drastically reduces the amount of code required to build large applications
• ASP.NET makes development simpler and easier to maintain with an event-driven, server-side programming model
• ASP.NET pages are easy to write and maintain because the source code and HTML are together
• The source code is executed on the server. The pages have lots of power and flexibility by this approach
• The source code is compiled the first time the page is requested. Execution is fast as the Web Server compiles the page the first time it is requested. The server saves the compiled version of the page for use next time the page is requested
• The HTML produced by the ASP.NET page is sent back to the browser. The application source code you write is not sent and is not easily stolen
• ASP.NET makes for easy deployment. There is no need to register components because the configuration information is built-in
• The Web server continuously monitors the pages, components and applications running on it. If it noticies memory leaks, infinite loops, other illegal software or activities, it seamlessly kills those activities and restarts itself
• ASP.NET validates information (validation controls) entered by the user without writing a single line of code
• ASP.NET easily works with ADO .NET using data-binding and page formatting features
• ASP.NET applications run fater and counters large volumes of users without performance problems

Differences between ASP.NET and Client-Side Technologies
Client-side refers to the browser and the machine running the browser. Server-side on the other hand refers to a ASP hosting Web server.
Client-Side Scripting
Javascript and VBScript and generally used for Client-side scripting. Client-side scripting executes in the browser after the page is loaded. Using client-side scripting you can add some cool features to your page. Both, HTML and the script are together in the same file and the script is download as part of the page which anyone can view. A client-side script runs only on a browser that supports scripting and specifically the scripting language that is used. Since the script is in the same file as the HTML and as it executes on the machine you use, the page may take longer time to download.
Server-Side Scripting
ASP.NET is purely server-side technology. ASP.NET code executes on the server before it is sent to the browser. The code that is sent back to the browser is pure HTML and not ASP.NET code. Like client-side scripting, ASP.NET code is similar in a way that it allows you to write your code alongside HTML. Unlike client-side scripting, ASP.NET code is executed on the server and not in the browser. The script that you write alongside your HTML is not sent back to the browser and that prevents others from stealing the code you developed.