In .Net/WPF, what is a Dependency Property and when should I use it?

Hi:

I'm a relative newbie to the world of WPF.
I've played a little bit and did a bunch of reading on dependency properties.

I've found and read many articles regarding Dependency Properties on the WEB,
but it still hasn't sunk in.

After all this reading, I'm still not entirely clear on what they REALLY are and when
I should REALLY BE USING them.

What is the relevence of using a PropertyMetaData() object when registering a Dependency Property?

I understand that if I declare a dependency property, then that property is associated
with an instance property on my class. I know that that property is then accessible
through XAML as a tagged element/property (<Property Foo> ... </PropertyFoo>)
rather than as if it were an attribute.

I also (sort of) understand where a dependency property gets stored when registered.

In the simplest terms (in ENGLISH), I'm still struggling with what it REALLY is and when to
use it.

Thanks,
JohnB