The .NET development platform is a set of software components for building both Web server applications and Windows desktop applications.

Applications created with the platform run under the control of the Common Language Runtime, or CLR (bottom), a software engine that loads applications, verifies that they can execute without error, checks for the appropriate security permissions, executes applications, and cleans up after they are finished. A set of class libraries supply code that enable applications to read and write XML data, communicate over the Internet, access databases, present user interfaces, and more. All class libraries rely on a library of base classes that provide functions for managing the most frequently used data types, such as numbers or text strings, and low-level functions, such as file input and output.

Web server applications normally rely on ASP.NET, a server-side library for processing Web requests. ASP.NET in turn relies on a Web service library for sending and receiving SOAP messages, and a Web user interface (UI) library (sometimes called Web Forms) for receiving user input from browsers and dynamically generating Web pages in response. Windows desktop applications can present a graphical UI through the WinForms library, also known as Windows Forms.

Finally, Visual Studio .NET provides a graphical integrated development environment (IDE) for creating applications on the platform. Developers write their code in one or more .NET programming languages, such as Microsoft's own Visual Basic .NET (VB.NET), Visual C++, Visual C# (pronounced "C-sharp"), and J# .NET (Microsoft’s implementation of the Java programming language on .NET). Numerous other .NET programming languages are available from third parties.