It's a program that allows you to set/make custom skins in windows, with special effects and different settings. The taskbar in my favorite skin, for example, is narrower, the buttons are smaller, and it's also a little transparent, letting the wallpaper be seen through lightly. Windowblinds skins tend to have more pleasant color schemes, too; muted and without that horrible metallic sheen/glare. There are even custom movies/gifs, like the one that shows up when you're moving folders and emptying the recycle bin, and they're generally better than the standard ones.

Someday I want to make a custom windowblinds skin as well, but it must take an insane ammount of work.

/pimping