What does a single apostrophe mean in Scala?

Question

In this slide show on ScalaActors.pdf what does the single quote indicate when the message is sent to the pong actor?

class Ping(count: int, pong: Pong) extends Actor {
def act() {
   pong ! 'Ping // what does the single quote indicate???
      receive {
         case 'Pong =>
      }
   }
}

Answer 1

It indicates a Symbol. Eg. cfr http://www.scala-lang.org/docu/files/api/scala/Symbol.html :

the Scala term 'mysym will invoke the constructor of the Symbol class in the following way: Symbol("mysym").

Answer 2

This defines a literal Symbol. See also this question.