A .resx file is a place where application developers can store resources that are used in their apps. By resources I mean things like text and images that appear in the UI.
Resources – An overloaded term
Resources can mean several things to a developer, especially in Silverlight. Several years ago when I would have a Resources directory in my ASP.NET app, and it would have subdirectories for my Images, CSS files and JavaScript files.
In a Silverlight project there are no CSS or JavaScript files (though there may be JavaScript files in the Web project), but you may still want subdirectories for Images and Styles (XAML files that provide custom style information).