RadControls for ASP.NET AJAX 
Built-in skins
These are the skins provided by Telerik. Since Q3 2011 they are embedded in the Telerik.Web.UI.Skins.dll assembly as web resources
(they were previously in the Telerik.Web.UI.dll assembly). To use a built-in skin the developer only needs to set the
Skin property of the control. Non-embedded versions of the built-in skins are provided in
"C:\Program Files\Telerik\RadControls for ASPNET AJAX \Skins" and in the /Skins/ folder inside the
RadControls installation ZIP archive. The complete list of embedded skins is available below:
Black
Default
Forest
Hay
Office2007
Office2010Black
Office2010Blue
Office2010Silver
Outlook
Metro
Simple
Sitefinity
Sunset
Telerik
Transparent
Vista
Web20
WebBlue
Windows7
Custom skins
These are skins created by the developer. They are not embedded in the Telerik.Web.UI assembly, so the control cannot register them automatically. If you use a custom skin you need to set the EnableEmbeddedSkins property of the control to false, as well as set the Skin property to the name of the custom skin. If you do not do so, an exception will be thrown saying that there is no embedded skin matching the Skin property:
Telerik.Web.UI.RadSlider with ID='RadSlider1' was unable to find embedded skin with name 'MySkin'. Please, make sure that you spelled the skin name correctly, or if you want to use a custom skin, set EnableEmbeddedSkins=false.
Modified built-in skins
It is possible to create a custom skin by taking an embedded one and making various adjustments. The non-embedded versions of the built-in skins are provided in the 'Skins' folder of the RadControls for ASP.NET AJAX install location. One can pick any skin for any control and modify some CSS rules. Then the resultant custom skin must be added to the website, the EnableEmbeddedSkins property of the control must be set to false and the modified CSS file must be manually registered in the ASPX page (or user control).
Non-skinned controls
If you do not want a predefined skin for a control, set its Skin property to "".
Base stylesheets
Usually, a RadControl has some collection of CSS styles, which should be applied regardless of the used skin (either built-in or custom). Such common CSS styles are included and registered on the page in a base stylesheet. In case you want to disable this automatic registration, set EnableEmbeddedBaseStylesheet to false.
Skin folder structure
A skin consists of a CSS file and images. Here is a typical skin folder structure:
The CSS files are named in the following manner: [Control].[Skin].css e.g. Slider.Inox.css.
The images are stored in a folder named after the control.
The base stylesheets (if present as non-embedded files) are stored in the parent folder of all skin folders and are named after the control - e.g. Slider.css