A skinwall tyre has a thing layer of rubber over the underlying threaded carcase of the tyre (on which the tread is bonded) on the sidewalls this allows the thread or "skin" to be visible, making them lighter. A gumwall is exactly that, a tyre that has a deep layer of yellow rubber over the entire carcase on which the tread is then bonded to, making them heavy.
Hope this helps.
Marty
