Each type of Window has a different shape and appearance. Drop-down listboxes look very different from buttons. Clearly, every subclass of Window should implement its own DrawWindow() method — but so far, nothing in the Window class enforces that they must do so. To require subclasses to implement a method of their base, you need to designate that method as abstract.
An abstract method has no implementation. It creates a method name and signature that must be implemented in all derived classes. Furthermore, making at least one method of any class abstract has the side effect of making the class abstract.
Abstract classes establish a base for derived classes, but it is not legal to instantiate an object of an abstract class. Once you declare a method to be abstract, you prohibit the creation of any instances of that class.