title: What's the difference between the atomic and nonatomic attributes?
body: <p>What do <code>atomic</code> and <code>nonatomic</code> mean in property declarations?</p>
  
  <pre><code>@property(nonatomic, retain) UITextField *userName;
  @property(atomic, retain) UITextField *userName;
  @property(retain) UITextField *userName;
  </code></pre>
  
  <p>What is the operational difference between these three?</p>