The Dot Net Base Class Libraries (BCL)

The Base Class Libraries (BCL) provides the fundamental building blocks for any application you develop, be it an ASP.Net application, a Windows Forms application or a Web Service. The BCL generally serves as your main point of interaction with the runtime. BCL classes include:

Namespace	Description
System	This namespace includes all the essential support you need for your programming, including base types (String, Int32, DateTime, Boolean, etc.), essential environmental support, and math functions, to name a few
System.CodeDom	all the support necessary to be able to create code, and run it, on the fly
System.Collections	The System.Collections namespace contains interfaces and classes that define various containers, such as lists, queues, bit arrays, hashtables and dictionaries.
System.Diagnostics	All the classes you need to diagnose your application, including event logging, performance counters, tracing, and process management APIs.
System.Globalization	This namespace includes fundamental support for Globalization, used throughout the rest of the Framework
System.IO	Includes fundamental Stream support which can be used by anyone, and then specifically targets the FileSystem (via File and Directory manipulation classes), SerialPorts, and Decompression
System.Resources	Used to allow an application to be translated into multiple languages, and then display the appropriate text based upon the current users language selection
System.Text	This namespace includes support for encodings, and Stringbuilder
System.Text.RegularExpressions	This namespace includes regular expression support, for robust parsing and matching of string data