title: What's the difference between the following state?
body: <p>maybe it's a little childish question, but I really want to know the detail. I've just seen this code:</p>
  
  <pre><code>@implementation SimpleMainViewController
  {
      SimpleTableViewController *simpleTableViewController;
      AboutViewController *aboutViewController;
  }
  </code></pre>
  
  <p>what's the difference between this and the following one?</p>
  
  <pre><code>@interface SimpleMainViewController : UIViewController
  @property(nonatomic,retain) SimpleTableViewController *simpleTableViewController;
  @property(nonatomic,retain) AboutViewController *aboutViewController;
  
  @implementation SimpleMainViewController
  @synthesize simpleTableViewController;
  @synthesize aboutViewController;
  </code></pre>
  
  <p>Thanks in forward.</p>