Cross-Domain Scripting Question

Cross-Domain Scripting Question

I'm using the IntenseDebate comments system. Like Google Adsense, you
add a couple of lines of javascript to your page, and that calls their
server, and inserts all kinds of controls and forms and links on your
page.

I'd like to provide a similar capability to sites which might like it
-- not comments, but something else where forms and data I provide
appear on their page via a few lines of Javascript that I provide.

I have no idea how this is done yet. Where can I read up on how to do this?

Thanks very much in advance to all for any info.