An application page is a web page that is deployed physically inside the SharePoint Root directory under the following path. C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\.An application page is different from a site page because it is not available in the content database.
Since application pages are compiled once, so it is faster
Example of Application Pages like settings.aspx, accessdenied.aspx.
An application page is a web page that is used in a SharePoint website. 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 SharePoint site. We can create application pages in Visual Studio by adding an Application Page item to a SharePoint project.
To create application page post Step by step Creating Application page in SharePoint 2013 Using Visual Studio
Thank you very much
Fahadullah Karimi
SharePoint Specialist