Namespaces in Unity 3D :

Namespaces are container of the classes. Namespaces help us to organize the scripts. When we create a new C# script in Monodevelop, two namespaces are included at the top, these are ' using UnityEngine; ' and ' using System.Collection; ' . We can access all the classes of these namespaces in our script. To put a class in namespace we need to surround the class with the namespace.

For Example : Use the keyword namespace and give any name to this namespace.

using UnityEngine;
using System.Collections;

namespace DemoNamespace
{
    public class DemoClass1 : MonoBehaviour 
    {
        void Start () 
        {
            Debug.Log("Namespace is created..");
        }
    }
}

There are three ways to use the classes of the namespace -

First : By using the 'using' directive at the top

using UnityEngine;
using System.Collections;
using DemoNamespace;

 public class DemoClass2 : MonoBehaviour 
    {
        void Start () 
        {
            Debug.Log("First way to use the classes of namespace.");
        }
    }

Second : Access the classes using dot operator

using UnityEngine;
using System.Collections;

 public class DemoClass3 : MonoBehaviour 
    {
        void Start () 
        {
            DemoNamespace.DemoClass1 MyClass = new DemoNamespace.DemoClass1();
            Debug.Log("Second way to use the classes of namespace.");
        }
    }

Third : To put the class in the namespace

using UnityEngine;
using System.Collections;
namespace DemoNamespace
{
    public class DemoClass4 : MonoBehaviour 
    {
        void Start () 
        {
            DemoClass1 MyClass= new DemoClass1();
            Debug.Log("Third way to use the classes of namespace.");
        }
    }
}

Thankyou !
Happy Coding !