[andrewmunsell]

They host a script, and provide an code to place on your website. You'll need to learn basic Javascript before you get to the point where you have a script that changes based on a user ID.

Hellobar (and a service I am working on) provide variable content based on an ID by requesting a "configuration" file from a server with the provided ID.

[materialhero]

Thanks for the quick response. Lets say for the sake of simplicity (and a short weekend project) that I want to have a <div> on my site that anyone can copy a code <script type="text/javascript" src="//www.mycrummysite/mydiv.js"></script> and put on their website so that it shows the my div. Every time i change the contents of the div, it also changes on their site. That seems like a good way to start. Is there a quick way to learn how to do this?

[andrewmunsell]

You can't do it that way-- once someone copies the code, it can never be changed by you. What you can change is the contents of the Javascript file. For example, when I make an update to my service, I upload the changes to script.js and the new code is run on the sites that embed that script.