The ExecuteUrl element in a Site Definitions ONET.XML can be used to run an ASPX file after a site is created.

This can be used if you need to do something programmatically to the site when it is created. For instance create a LookUp field in a list, this can not de done in the Site Definition and will have to be done manually or programmatically after the site is created.

Windows Sharepoint Services (WSS)

The ExecuteUrl element only works for Team Site Templates. It only works for Meeting Workspace and Document Workspace templates when the sites are created from the Create function in Sharepoint, E.G. using create.aspx
the ExecuteUrl element is ignored if a Meeting Workspace is created using the checkbox when creating an event in an event list.

The ExecuteUrl element is also ignored if a Meeting Workspace is created from Microsoft Outlook 2003.

Creating a Document Workspace from the dropdown list on a document library will also NOT trigger the ExecuteUrl element, nor will create a Document Workspace from Microsoft Word.

Microsoft Sharepoint Portal server

The ExecuteUrl element will also be ignored when used in the ONET.XML for a Sharepoint Portal Server 2003 Area definition.

Using the ExecuteUrl element

The ExecuteUrl parameter is located in the Configuration section in ONET.XML.
You can create an ASPX application that can be run when a Site is created using your Sharepoint Site Definition.
The ASPX file should be placed in Local_Drive:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\60\TEMPLATE\LAYOUTS
Place a ExecuteUrl element in ONET.XML pointing to you ASPX file like this:

This page will then be run when a site is created using this site definition.

You can find some information about the ExecuteUrl element and ONET.XML in the Microsoft Sharepoint Products and Technologies SDK