Quote
Usually that dialog is another set of HTML markup that exists on the page and is "hidden" via CSS or is constructed dynamically when the button is clicked. This HTML tends to have CSS styles that position it correctly on the page, and other js to handle the form submit or button press after you finish typing in the box. There is nothing special about these popups, they are just more "markup" that happens to be floating above other markup instead of set inside the flow of the page.
Maybe this is common knowledge to web designers but I'm fascinated both by how this was explained as well as why there are no services currently providing this for casual users in Netvibes-level form.
from: Quora