GUI Skin Asset

So I’m pretty new to Unity and I’ve been learning about GUI scripting and how you can make Menus and all sorts of other things with that. I feel that I’m fairly confident in scripting a GUI to fit what I need, and get it to show.

However I’ve found something that I know nothing of, and I have found nothing so far explaining what it does or how you use it.

What I’m referring to is a GUI Skin Asset that you can create by doing Assets → Create → GUI Skin. Or Just simply Create → GUI Skin in the Project Panel.

Could someone explain to me in detail what this is exactly and how it works? Or point me to a documentation on it?

Thanks in advance, =)