Extensions vs categories

Hello, according to Apple do***entation "The Objective-C 2.0 
Programming Language (pag 43)", extensions are a way to differentiate 
between a public API and an API for the class of the framework. IMO 
categories also accomplish the same requirement. So my doubts are:

1. Are extension a new feature in Objective-C 2.0, because it's not 
present in previous releases of "The Objective-C Programming Language".
2. Where is the conceptual difference between categories and 
extensions. Or is just a question of flavors.

Regards.