from http://dmitrysoshnikov.com/ecmascript/chapter-7-2-oop-ecmascript-implementation/ -- good explanations across the board

toString, ValueOf

var a = {
  valueOf: function () {
    return 100;
  },
  toString: function () {
    return '__test';
  }
};
 
// in this operation
// toString method is
// called automatically
alert(a); // "__test"
 
// but here - the .valueOf() method
alert(a + 10); // 110
 
// but if there is no
// valueOf method, it
// will be replaced with the
//toString method
delete a.valueOf;
alert(a + 10); // "_test10"