The .NET framework class library, as the name suggests, is a library of classes, interfaces and value types. The applications, components and controls for applications are built on this framework and it provides the developer the access to the system functionality. In other words, the classes and structures can be leveraged as base building blocks for application development. These classes are often described as an API and form a boundary interface between the application and the operating system. Though the concept is not new to Visual basic Developers who have been using the ADO library, the Win 32 API and COM + services, it forms a massive code base on which the application can be built.

The .NET Framework class Library is organized into namespaces. The namespace is a container for functionality. Similar classes and constructs are grouped together in a namespace to define parent-child relationships. Namespaces can be nested into namespaces.

All namespaces stem from the root namespace called System Namespace. It contains all data types including the Object data type. Though all namespaces are subordinated to the System namespace, User defined libraries can also coexist with the System namespace. They can have their own root namespace which can be language focused namespaces such as Microsoft.Csharp, Microsoft.VisualBasic.

The most significant feature of the .NET framework is the class Library collection of reusable types can be integrated with CLR. The programmer can accomplish a range of common programming tasks, such as string management; data collection; data base connectivity and file access using the .NET framework class library. The developer can create console applications, Windows GUI applications, ASP.NET applications, XML Web services or Windows services.