Winterboard is an application developed by Jay Freeman (The creator of Cydia.)
It allows you to theme your device. From Icons to status bars, from the lockscreen to the power-off screen, you can do pretty much anything in Winterboard.
In Cydia, there are thousands of Themes for Winterboard. You can simply install any theme and Winterboard will also install. Once you are ready to set it up, launch Winterboard from the SpringBoard (home screen).
Once you are in Winterboard, press “Select Themes.” From there you can enable and disable themes. You can also rearrange the themes’ priority. For example, you have a theme that changes the icons and the slide to power off text. You have another theme that only changes the slide to power off text. If you put the second theme in front of the first one, the slide to power off text will be of the second theme. If they were swapped then ths slide to power off text would be of the first theme.
My opinion is that it is a great app.
Here are some images of some Winterboard themes:
GinoAwesome theme (You need to add the source “http://apt.pwncenter.com” to get it.)
Wood Shelves theme