bar: Int
This is barely a constructor parameter. If this variable is not used anywhere except the constructor, it remains there. No field is generated. Otherwise private val bar field is created and value of bar parameter is assigned to it. No getter is created.
private val bar: Int
Such declaration of parameter will create private val bar field with private getter. This behavior is the same as above no matter if the parameter was used beside the constructor (e.g. in toString() or not).
val bar: Int
Same as above but Scala-like getter is public
bar: Int in case classes
When case classes are involved, by default each parameter has val modifier.