title: When is it better to use an NSSet over an NSArray?
body: <p>I have used NSSets many times in my apps, but I have never created one myself. </p>
  
  <p>When is it better to use an <a href="https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSSet_Class/" rel="noreferrer"><code>NSSet</code></a> as opposed to an <a href="https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSArray_Class/index.html#//apple_ref/doc/c_ref/NSArray" rel="noreferrer"><code>NSArray</code></a> and why?</p>