What is internal and what is the use of internal in OOP

Internal is access modifier. Methods and properties designated as such are accessable by all objects within the same package only.  I believe that's the defination.  When writing classes you modify the access to different properties and methods using access modifiers such as public for publiic interface, private for use by the class it's in only etc.  This link explains better http://lassieadventurestudio.wordpress.com/2009/03/31/public-internal-protected-and-private/  I'm just beginning to learn OOP myself so I can't offer much help.