Creating an Application Page in Windows SharePoint

You can create custom application pages to add user interface components to a custom solution based on Microsoft Windows SharePoint Services 3.0. Unlike site pages (for example, default.aspx), a custom application page is deployed once per Web server and cannot be customized on a site-by-site basis. Application pages are based in the virtual _layouts directory. In addition, they are compiled into a single assembly DLL. They are also used across all sites within a server farm. For these reasons, they perform better than site pages. With application pages, you can also add inline code. With site pages, you cannot add inline code

<%@ Assembly Name="Microsoft.SharePoint, [full 4-part assembly name]"%> 
<%@ Page Language="C#" MasterPageFile="~/_layouts/application.master" 
         Inherits="Microsoft.SharePoint.WebControls.LayoutsPageBase" %>

<%@ Import Namespace="Microsoft.SharePoint" %>

<script runat="server">
  protected override void OnLoad(EventArgs e) {
    SPWeb site = this.Web;
    lblSiteTitle.Text = site.Title;
    lblSiteID.Text = site.ID.ToString().ToUpper();
  }
</script>

<asp:Content ID="Main" runat="server"
             contentplaceholderid="PlaceHolderMain" >
  Site Title: <asp:Label ID="lblSiteTitle" runat="server" />
  <br/>
  Site ID: <asp:Label ID="lblSiteID" runat="server" />
</asp:Content>

<asp:Content ID="PageTitle" runat="server" 
             contentplaceholderid="PlaceHolderPageTitle" >
   Hello World
</asp:Content>

<asp:Content ID="PageTitleInTitleArea" runat="server"
             contentplaceholderid="PlaceHolderPageTitleInTitleArea" >
   The Quintessential 'Hello World' of Application Page
</asp:Content>