Question about S4

Dear R-ers,

I don't understand the following, maybe someone will help me 
explain:

setClasss('A')

[1] "A"

new('a')

Error in new("a") : 
  trying to generate an object from a virtual class ("a")

setClass('b', contains='a')

[1] "b"

new('b')

An object of class ?b?
<S4 Type Object>

In what way is B more concrete than A so that it's possible do 
instantiate B but not A? I don't quite get it. B adds nothing to 
nothing, and yet it's instantiable, while it's base is not. Makes 
no sense to me.

-- Hun