ASP.Net is a microsoft develpment that is part of the .Net framework. Do not confuse it with ASP as they are two completely different things that happen to share part of their name in common. ASP.Net requires the use of a programming language such as VB or C# so that you can program a dynamic website via the server (that is because both VB and C# are languages used with the .Net framework). However, you have the option of creating server side scripts using PHP or ASP, but those two languages do not use the .Net framework and can be a bit more time consuming to use in web development.
You should use ASP.NET instead of classic asp because: 1.ASP.NET gives Ability to use true object-oriented design for programming pages and controls . 2.If an ASP.NET application leaks memory, the ASP.NET runtime unloads the AppDomain hosting the erring application and reloads the application in a new AppDomain. 3.Session state in ASP.NET can be saved in a Microsoft SQL Server database or in a separate process running on the same machine as the web server or on a different machine. That way session values are not lost when the web server is reset or the ASP.NET worker process is recycled. 4.An extensive set of controls and class libraries allows the rapid building of applications, plus user-defined controls allow commonly-used web template, such as menus. Layout of these controls on a page is easier because most of it can be done visually in most editors. Hope this help
ASP.NET makes building real-world Web applications dramatically easier with server controls that let you build great pages with far less code than classic ASP. ASP.NET supports not only Microsoft Visual Basic Scripting and Microsoft JavaScript but also more than 25 .NET languages, including built-in support for Visual Basic .NET, Microsoft C#, and JavaScript .NET.