IT is a collection of classes includes methodes and variables.
Collection of classes
Namespace is collection of classes Like package in java
namespace is like a packege
In C#, Collection of classes is known as Namespace.
Here....http://www.c-sharpcorner.com/article/namespaces-in-C-Sharp/
It is a container for the Classes. We use namespaces for grouping the related classes. "Using" is the keyword used for using Namespace.