Here some clarifications(I'll try)
The code posted above will only work for IE5+ browsers.
Here are answers
1. What is this? Is it a complete file, message, html, or what?Where does this script go? How does it work?
The script is a good piece of script that opens a stream to the given destination,where destination will be "SomeURL"
in the line below
2. Is "Some URL" my recipicant's ".dtd" or just a regular ".html"?
This "someurl" will be the address where you want to send the XML file.
In my pinion you are not understanding it correctly.I am sure when some one said about sending a file that means it should be residing at your server and with a given URL people can access it.
What does a .dtd have anything to do with this??. adtd is a totally separate thening.That actually controls how the XML file will look like(eor e.g. what will be the names elements and what data type will they contain).
3. Do I write the 4th line as exactly like this?
the fourth line assumes that you already know how to create and XML dataisland object, you create that and then you pass that do the send method.
Make sure you understand the concept that what exactly is you require to do.