Please find below details that i found at dot net spider...
Skins-A set of properties that define a control's appearance
Cascading style sheet (CSS)-A standard for defining the appearance and layout attributes of HTML elements
Images-Files that define the site's appearance, such as company logos
A theme has, at minimum, a skin file. Some overlap occurs between skins and CSS because both are used to control appearance. They have some crucial differences, however:
Skins don't cascade. Unlike with CSS, where you can create a hierarchy of styles, you define one skin for each type of control you use on your site. See the sidebar "Cascading styles" for more details on how CSS styles cascade.
Skins define properties for ASP.NET Web server controls. CSS styles apply to HTML elements.
Skins apply to a single Web server control. Styles can apply to a single HTML element or to collections of elements.
Styles can be used to control the positioning of elements on a page.
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