class Simple 
{
    public int a;
    public void b()
    {
    }
}
Simple s = new Simple();

s.a = 6;   // assign to field a;
s.b();     // invoke member function b;

// The class Console in namespace System:
System.Console.WriteLine("hello");

using System;
// ...
System.Console.WriteLine("hello");
Console.WriteLine("hello");   // same thing

using System;
// A namespace containing another Console class:
using OtherSystem; 
// ...
// Must qualify Console: 
System.Console.WriteLine( "hello" );