Lightbox is a jQuery/CSS technique used to display additional content instead of using a popup. The inserted content is displayed in a portion of the browser window, and the rest of the window is dimmed out.
Many sites use this technique. A good example is Facebook which uses Lightbox techniques when displaying images.
This page demonstrates a simple form of lightbox developed by by Kyle Schaeffer at kyleschaeffer.com.
I tested in Chrome, IE, Firefox, and Opera browsers.