An application page is an ASP.NET Web page that is designed for use in a SharePoint Web site. Application pages are a specialized type of ASP.NET page. The primary difference between an application page and a standard ASP.NET page is that an application page contains content that is merged with a SharePoint master page. A master page enables application pages to share the same appearance and behavior as other pages on a site.
Questions having the tag application-pages regards the design, branding, implementation and maintainenca of Application Pages.
Reference: Creating Application Pages for SharePoint