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When to use an interface instead of an abstract class and vice versa?

Hi, I am teaching OOP concepts to non-programmers. I wanted to know how can you explain the difference between an interface and an abstract class.
What I am actually looking for, is a real world example that can help highlight the difference between the two.

Answer 1

The player Interface

In my Java courses I often use this kind of image and ask: "What is this ?"

Every time someone will say "that’s a player". From this image you can teach anybody what an interface is. This Interface allow any user to "play" something. Everybody knows what these buttons mean, even if you don’t know what exactly will be done, you can use anything with this interface and you know that the little arrow will "play" and other arrows will probably send you forward or backward.
Everything that will have those buttons will provide a standard behavior that any user will know before even starting to use them.

I usually try to avoid the "contract" word which can be misunderstood.

The DVD player Abstract class

And then from the Play Interface, I go to the VCR (or DVD) player. Every constructor of DVD player must add some special functions to transform a simple unknown player into a DVD player. For example the eject button. And they must correctly implement Player.
The play button will launch the content of the DVD.

But even if DVD Player provide the basic behavior of a DVD player, not everything is done. You can’t simply have "a" DVD player, it has a brand and most of the time it has its own firmware. A this time you’ll need to extend the DVD Player abstract class to add your own little components.