How to get this popup window style?

If you go to http://www.sharethis.com/ and click on the green shareThis widget next to the text 'Try it out', you'll see a window pop-up.

I've noticed everyone is beginning to use a pop-up window with this style.

What's this? Is 开发者_如何学Pythonit just a div or a new browser window? How to achieve this effect?

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Check out any one of these:

• Lightbox
• Lightbox2
• Fancybox

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I guess the popup-window is actually two divs, one which darkens the site, one for the window. Also guessing: both could use the "opacity" and "z-index" css attributes.