An abstract class is an abstraction. It guarantees behaviors exist but does not force how the behavior is implemented. This leaves you free to change the way the behavior is implemented, if you later decide you want to.
An abstraction is like a keyboard, monitor, or cell phone. All keyboards have the ability to enter data; all monitors have the ability to display pixels; and all cell phones have the ability to make calls. But the manufacturers of these items have different ways of implementing the behavior.
So when you want to make a call, you can do it from pretty much any cell phone because all cell phone manufacturers create phones that adhere to the common abstract idea of what a cell phone is. You don't need to relearn how to make a call on a Samsung if you have already learned how to do it on a BlackBerry or LG.
A cell phone is a cell phone, and subclasses of an abstract class are all the abstract class.
