.NET Framework Class Library
The Framework Class Library (FCL) is the standard set of types required to run .NET applications and is included with the .NET framework.
These types are spread over several assemblies and in different namespaces. The most important namespace is the System namespace, which contains the classes for base types like Int32, Int64, Double, String and Char. The System.Collections and System.Collections.Generic namespaces contain structures like lists and dictionaries, which are essential for application development.
Other namespaces include the System.IO for reading and writing to files and other streams, System.Net for socket support or System.Reflection for reading and generating assemblies on the fly, and so on.