Easy enough to show how it works in one post, but you're right, it's tough to explain all the concepts all at once. There's a lot involved for the whole process, even if it boils down to ~10 lines of code. You need some knowledge of html/css, javascript/jQuery, a server-side scripting language, and JSON. I don't know what type of experience WebWatcher has with each of these, but here's an example that goes through the whole thing:
http://jsfiddle.net/pr4bB/2/
So, given a javascript object, you can encode the object into a JSON string. You can then send the JSON string to a server-side script of yours, process it however you want, and send a response back to your page.
In the example I posted, here's what happens:
1. A generic javascript object is created.
2. A JSON string is generated for the object using a jQuery add-on (
http://code.google.com/p/jquery-json/).
3. When the user clicks the button, the JSON string is POSTed to a server-side script.
4. A response is given (in this case, the script doesn't return anything).
5. A modal window is created (using jQuery UI), and the response from the script is appended.
And if you're just loading content from another page, it's even easier:
http://jsfiddle.net/xGaZL/2/
jsfiddle doesn't really have any pages that work for loading, so you just have to pretend the modal has content from another page haha)