using (C# Reference)
The using keyword has two major uses:
As a directive, when it is used to create an alias for a namespace or to import types defined in other namespaces. See using Directive.
As a statement, when it defines a scope at the end of which an object will be disposed. See using Statement.
See Also
Reference
C# Keywords
Namespace Keywords (C# Reference)
extern (C# Reference)
Concepts
C# Programming Guide
Namespaces (C# Programming Guide)