Nested just below the System namespace are a number of second-level namespaces, which contain such classes as:
Contains classes representing the elements and structure of a source code document.
Contains interfaces and classes that define various collections of objects, such as lists, queues, arrays, hashtables, and dictionaries.
Contains classes that are used to implement the runtime and design-time behavior of components and controls.
Contains classes that allow the creation of custom installers for software components.
Consists mostly of the classes that constitute the ADO.NET architecture and are used for database connectivity.
Contains classes that allow debugging of applications and code tracing.
Contains classes that provide access to the Active Directory from managed code.
Contains classes that provide access to GDI+ basic graphics functionality. (More advanced functionality is provided in the System.Drawing.Drawing2D, System.Drawing.Imaging, and System.Drawing.Text namespaces.)
Contains classes that allow synchronous and asynchronous reading from and writing to data streams and files.
Contains classes that provide a simple programming interface to many of the common network protocols, such as FTP and HTTP. (The System.Net.Sockets namespace provides lower-level network access control.) ...