"As an alternative to subclassing, Objective-C categories provide a means to add methods to a class. What’s intriguing, is that any methods that you add through a category become part of the class definition, so to speak. In other words, if you add a method to the NSString class, any instance, or subclass, of NSString will have access to that method.
Defining a category is identical to defining the interface for a class, with one small exception: you add a category name inside a set of parenthesis after the interface declaration..."
Categories and Extensions leiras az iPhone OS Reference Libraryban
