title: Explain the difference between mutable and immutable
body: <p>What is the difference between mutable and immutable?</p>
  <p>Such as:</p>
  <p><code>NSString vs NSMutableString.</code></p>
  <p><code>NSArray vs NSMutableArray.</code></p>
  <p><code>NSDictionary vs NSMutableDictionary.</code></p>
  <p>What is the difference between a mutable object and the other object [which I assume is immutable]?</p>