Microsoft .NET is a software development platform based on virtual machine based architecture. Dot net is designed from the scratch to support programming language independent application development. The entire .NET programs are independent of any particular operating system and physical hardware machine. They can run on any physical machine, running any operating system that contains the implementation of .NET Framework.

The core component of the .NET framework is its Common Language Runtime (CLR), which provides the abstraction of execution environment (Physical machine and Operating System) and manages the overall execution of any of the .NET based program. With dot NET, Microsoft introduces a completely new architecture for Windows applications (WinForm), Data Access (ADO.NET), Web Applications (ASP.NET), Windows components (Assemblies), Distributed Applications (.NET remoting),and above all the XML based Web Services.

• Assemblies: An assembly is either a .DLL or .EXE that forms a part of an application. It contains MSIL code that is executed by CLR. The following are other important points related to an assembly
1. It is the unit on which permissions are granted.
2. Every assembly contains a version
3. Assemblies contain interfaces and classes. They may also contain other resources such as bitmaps, file etc.
4. Every assembly contains assembly metadata, which contains information about assembly. CLR uses this information at the time of executing assembly.
5. Assemblies may be either private, which are used only by the application to which they belong or Global assemblies, which are used by any application in the system.
6. Two assemblies of the same name but with different versions can run side-by-side allowing applications that depend on a specific version to use assembly of that version.

The four parts of an assembly are:

• Assembly Manifest : Contains name, version, culture, and information about referenced assemblies.
• Type metadata : Contains information about types defined in the assembly.
• MSIL : MSIL code.
• Resources : Files such as BMP or JPG file or any other files required by application.
• Common Type System: Common Type System (CTS) specifies the rules related to data types that languages must follow. As programs written in all languages are ultimately converted to MSIL, data types in all languages must be convertible to certain standard data types.
CTS is a part of cross-language integration, which allows classes written in one language to be used and extended by another language.
• Cross-language Interoperability:.NET provides support for language interoperability. However, it doesn?t mean every program written in a language can be used by another language. To enable a program to be used with other languages, it must be created by following a set of rules called Cross Language Specifications (CLS). Cross-language inheritance is the ability to create a class in C# from a class created in VB.NET. When an exception is raised by a program written in C#, the exception can be handled by VB.NET. This kind of exception handling is called cross-language exception handling. .NET has brought a set of new features which are to be understood by every programmer developing applications for Windows. There is no way any Windows programmer can ignore .NET, unless he is desperate to be outdated. Microsoft will provide .NET as part of its operating systems in future releases. It is the platform for programmers. It is not new OS from Microsoft or a new language. It is the environment for which you develop applications. It is rich in terms of features.

1. Windows applications: typical Client/Server applications.
2. Web applications: Web sites and Intranet applications.
3. Web services: Programs that are accessible from anywhere using universal protocols like HTTP and SOAP.
4. Console Applications:Simple console based applications without any GUI. Run from command prompt. Best suited to learn fundamentals and also for applications such as server sockets.
5. Mobile Applications: Contain web pages that run in mobile devices such as PDA's (Personal Digital Assistant) and Cell phones.