DotNetNuke Skinning Whitepaper (Skin definition section)

The definition of skin can be created by way of: HTML or ascx (user control). If you're a asp.net and don't know much about web designers, HTML is the best choice. If you are more adept at asp.net and are planning to create skin with tools such as vs.net, then ascx is your first choice. Basically, the difference between the two methods is only the skin definition file suffix and the corresponding user control identifier (please refer to Appendix A for details)

Generally a skin bag requires a minimum of two skin files, a layout to define the site's foreground, and a layout to define the site's background management. The front desk typically has a wide variety of content controls and different layout columns, and the latter management interface has only a few management controls placed in a separate column.