Very simple but annoying question about differentiation

[Fraggr]

There are different greek letters when differentiating.

I know the d/dx meaning, Δ/Δx meaning, the letter for partial derivative differentiation meaning (like in this image http://upload.wikimedia.org/math/1/1/6/1167391da9a316b25ffd67f3bffc0bef.png), but the lower case delta is the one I don't understand. What does it mean?

 

[Fraggr]

That's the kinda backwards 6 one in the image.

I've seen it crop up several times and I've never really understood what it is :s