Hello Waleed Seada,
A reference of the CSS selectors can be found in the online help for RadControls for ASP.NET AJAX, in the Appearance and styling > CSS Skin File Selectors articles. There are specific files for each RadControl (after all, each has its own CSS class prefix) - the following article is for RadPanelBar
The classes can be used outside the RadControls, but all of them are targeted using the .RadControlName class (.RadPanelBar, for example).
the Telerik team