difference between def/val assignment

Hi all,

running the following code yields

null
null
null
new value

class Data {
     var value: String = _
}

object DataTest {
     def main(args: Array[String]) {
         def a = new Data
         val b = new Data

         println(a.value)
         println(b.value)

         a.value = "new value"
         b.value = "new value"

         println(a.value)
         println(b.value)
     }
}

Can anyone explain the difference between def and val in this  
situation? Playing around with both I cannot find any difference  
between 'a' and 'b', they are both the same class. But 'a' must be  
something different. What exactly is it then?

Cheers,
Christoph