title: Swift 2: !, ? -" Value of optional type "..." not unwrapped"
body: <blockquote>
    <p>Value of optional type "..." not unwrapped; did you mean to use '!' or '?'?</p>
  </blockquote>
  
  <p>Can someone explain me what this error message exactly means? When should I use '?' and when '!'?</p>