This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
The first one is an empty reference - a null reference if you will. So no, you can't call orderList.add(order); on it because you would get a NullPointerException, (unless you assign an object to it between the variable definition and method call).
The second one actually creates an Object of a suitable, concrete implementation of List, and so can have a method called on it without an error occurring.