Access Modifiers (C# Reference)

Public :

 Access is not restricted .
               
 Visibility modifier that makes members of a class or interface visible to any code that has access to the class or interface.

Internal :

Access is limited to the current assembly  with in the Files .
Visibility modifier that makes a class, interface, or member visible only to the current package.

private Modifier

Visibility modifier that makes a class member visible only to members of the same class.

protected Modifier

Visibility modifier that makes a member of a class or interface visible only to the current class or interface and derived classes of the current class.

         
The protected modifier makes a member of a class or interface visible only within that class or interface and all derived classes of the current class. Code outside the current class cannot access protected members.