Classes represent kinds of things, whether they be concrete things, graphical things, or conceptual things. And instances of classes represent specific things. For example, in JavaFX:

class Shoe {
   var color : String;
   var size: Number;
   var style : String;
}

def rubySlipper = Shoe {
   color: "red"
   size: 7
   style: "slipper"
}

Here the class Shoe represents a kind of thing, and the instance stored in rubySlipper is a representation of a concrete thing. Note that there are instance variables that store the state of the instance.

Whereas the class CheckBox is a kind of graphical thing:

public class CheckBox extends CustomNode, Resizable {
   public var title : String;

   public var visible : Boolean;

   public var skin : Skin on replace oldSkin {
      if (skin != null) then skin.control = this;
      if (oldSkin != null) then oldSkin.control = null;
   }

   public function requestFocus() : Void {
      if (visible) then skin.requestFocus();
   }
}   

It extends two other classes (CustomNode and Resizable), therefore including their instance variables and instance functions. CheckBox adds three more instance variables, including skin which has an on-replace clause to maintain a relationship between the skin and the CheckBox. It also adds an instance function requestFocus.

[To do: add a section on instance initialization (before the instance variable initialization section). Then clean up the instance variable initialization section]