Application pages in SharePoint 2010 is a very useful concepts.
-These pages will be available to all sites of a farm.
-They are deployed under C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS folder.
-They are available through a virtual directory namely _layout.
-These pages are basically for administrative tasks.
-To create an application page using Visual Studio 2010 is very easy. By using the template Application Page you can create an application page.
-By default the page will use the default.master page.
-The page is derived from the base class LayoutsPageBase which is derived from Microsoft.SharePoint.WebControls namespace.
-Application pages cannot be personalized or customized by the end user because they are defined on the file system.
-Example of application page is settings.aspx.