Category Pros and Cons
A great primer into Objective-C categories by Mark Dalrymple for The Big Nerd Ranch:
Objective-C categories are cool. They allow you do something that you can’t do in most compiled languages: add new methods to existing classes. You can even add methods to classes that you didn’t write.
Suppose you wrote some code to translate an ordinary string, like “Hello, perhaps I could enjoy a bit of your delicious sandwich over there?” to LOLcat speak, like “I can has cheezburger?”