*Hoping this hasn't already been asked*

I'm a little confused about the whole concept of Dark Magic.

I'm assuming that the following are examples of Dark Magic:

Unforgivables
The potion that had to be drunk to get the locket Horcrux
The necklace that Katie Bell touched
'Sectumsempra'

But what makes something 'Dark'? A definition of Dark Magic would be awesome.

And plenty of examples, if possible.

~Midnight Storm