Namespaces of .Net

System: Includes essential classes and base classes for commonly used data types, events, exceptions and so on
System.Collections: Includes classes and interfaces that define various collection of objects such as list, queues,
hash tables, arrays, etc
System.Data: Includes classes which lets us handle data from data sources
System.Data.OleDb: Includes classes that support the OLEDB .NET provider
System.Data.SqlClient: Includes classes that support the SQL Server .NET provider
System.Diagnostics: Includes classes that allow to debug our application and to step through our code
System.Drawing: Provides access to drawing methods
System.Globalization: Includes classes that specify culture-related information
System.IO: Includes classes for data access with Files
System.Net: Provides interface to protocols used on the internet
System.Reflection: Includes classes and interfaces that return information about types, methods and fields
System.Security: Includes classes to support the structure of common language runtime security system
System.Threading: Includes classes and interfaces to support multithreaded applications
System.Web: Includes classes and interfaces that support browser-server communication
System.Web.Services: Includes classes that let us build and use Web Services
System.Windows.Forms: Includes classes for creating Windows based forms
System.XML: Includes classes for XML support