namespace

A namespace is a way of organizing your classes or other types.

They also help to resolve name clashes. If two classes in different namespaces have the same name, then you can qualify the class name by its namespace (i.e. namespace.classname) so the compiler knows which one you mean. 

Namespaces can be nested within each other.

The classes or other types in a particular namespace can be imported into a C# source code file by means of the 'using' directive:

using MyNamespace;

This means that all classes in MyNamespace can be used without qualification unless, of course, you need to resolve a name clash.