Represents any page stored in the _layouts virtual directory in SharePoint Products and Technologies.
Real World Example
A developer must create a custom page display in the site administration area for all site collections. She develops a new page that is stored in the _layouts directory and that will be called by a new custom action.
The _layouts directory hosts application pages, which contain general administration and settings for SharePoint Products and Technologies. Pages in the _layouts folder operate much like any ASPX page, for example, allowing inline code. However, they cannot contain Web Part zones.
The _layouts folder refers to the following file system location:
%COMMONPROGRAMFILES%\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS
Pages stored in the _layouts folder are available globally on the server