Take a look at this class hierarchy:
Using a single class inheritance mapping strategy, you'd have a talbe that looks like this:
You'd get a database that looks something like this, after a single instance of each class was created:
Alternatively, a joined table inheritance mapping would generate tables that look like this:
And a single table per class mapping strategy would give you tables that looked like this:
Sorry about posting such big images. I just think that this visually helps you see how mapping strategies differ in how the data is managed.
These images were taken from this tutorial on Inheritance Mapping with Hibernate and JPA Annotations:
Mapping Inheritance with Hibernate and JPA - An Example -Cameron McKenzie