Site pages vs Application pages

Site pages are pages that are created, edited, and customized by end users. They are primarily used for the content in a site. Site pages come in two types—a standard page and a Web Parts page. A standard page contains text, images, Web Parts, and other elements. A Web Parts page contains Web Parts in Web Part zones. They have a predefined layout that uses Web Part zones. Both types of site pages are edited using a Web browser or Microsoft SharePoint Designer.

Site pages are provisioned from a template page that is stored on the file system of the front-end Web server. When a site is provisioned, SharePoint Foundation creates a pointer to the instance of the page template on the file system. This allows SharePoint Foundation to avoid repeatedly creating copies of the pages, which are provisioned each time a site is created.

When a user customizes a site page, the template for the page is then stored in the content database. The page is retrieved from the content database every time it is requested by a user. A customized page can, however, be reset to the original template page through the Web browser or a tool such as SharePoint Designer.

Application pages are used to support application implementations in SharePoint Foundation. Application pages are stored on the file system of the front-end Web server in the %ProgramFiles%\Common Files\Microsoft Shared\web server extensions\14\TEMPLATE\LAYOUTS directory and exist for every site in a Web application. This folder is mapped to an Internet Information Services (IIS) virtual directory called _layouts. Every site and subsite will have access to the application pages by using the _layouts virtual directory. For example,http://myserver/_layouts/settings.aspx and http://myserver/subsite/_layouts/settings.aspx access the same application page on the front-end Web server unlike site pages, which are an instance for the specified site.

Application pages are not subject to the same restrictions as site pages. They allow in-line code without restriction. They cannot, however, use dynamic Web Parts or Web Part zones or be modified using SharePoint Designer. Modifying the default application pages is not supported in SharePoint Foundation. Custom application pages can be added to a subdirectory inside the _layouts folder.