Extension Description
.asax The Global.asax file defines application-level event handlers.
.ascx Defines a user control. User controls are used to encapsulate a block of reusable user interface functionality.
.ashx Defines a custom HTTP handler. Handlers are used to implement custom response functionality based on the extension of the request.
.asmx Defines an XML Web service.
.aspx Defines a Web Form.
.axd Special handlers used to manage Web site administration requests.
.config An XML-based configuration file (named Web.config) for an application or for the machine itself (machine.config).
.master Defines a master page that specifies the common layout for other Web Forms.
.resx Resource file containing resource strings used for localization.
.sitemap Defines the navigation structure of the site.
.skin Defines the visual property settings to be applied to controls in a site's theme.