DotNetNuke skin objects are ascx control (generally found at ~/Admin/Skins folder) . Skin objects are meant to be reusable UI elements that helps showing a single repeating elements of the page.
For instance, If are running a portal of dotnetnuke and want to show up a latest announcement entry in all your parent and child portal users, you can create a skin object that shows that entry and place that skin object to the skin.This way you can simply integrate the display of the latest announcement for all users in smart way using skin objects.
I worked on several skin objects. here is a quick list:
skin object to display custom logo image selected by an individual portal admin. They can upload the image and the change reflects to the entire portal pages
Vertical navigation which contains custom set of menu for advertising and control panel
News and blog feed display
And many more
We can also show/hide the skin object dynamically. For instance, if you like to hide the skin object when there is no announcement for last 2days, its easily possible to do in it.
Skin object offers smart way to reuse your design components.
There are existing skin objects available with dotnetnuke which provides you some interesting features that you can easily integrate and reuse. Skin objects like logo, user, login, breadcrumb, dnnmenu are almost useful in all the dnn sites. Eagleworx shows a complete list of skin objects and the properties provided by them. You can download the latest skin object documentation from download section of dotnetnuke.