title: What means layer and not layer in customviews?
body: <p>if i create a custom view class that inherit from the <code>UIView</code> Class i can access properties with the keyword <code>layer.</code> for example <code>layer.cornerRadius = 10</code> and for example the background with <code>backgroundColor = (Color Literal)</code> WITHOUT <code>layer.</code> </p>
  
  <p>Why? What means <code>layer</code> and when i use it and when not?</p>