With modal dialogs (overlays), users don’t have to deal with multiple windows. When a modal window opens, it opens inside the current page and users don’t have to deal with extra windows popping up.

When a modal dialog is shown, the content beneath the overlay cannot be acted upon until the overlay is dismissed. Modal overlays don’t allow you to refer back and forth between two sources of information, or move fluidly between two actions.

Today it is good practice to darken the parent window to provide a visual indicator to the user that the main window is inactive. This technique is usually called Lightbox.

The following list provides some links to blogs about the con’s and pro’s of modal overlays:

• Modal windows : about.com Javascript
• Why Modal Windows Have Killed Popup Windows, by anthony, UXMovement
• Mobile Firefox and Designing Without Modal Overlays, by Aza Raskin
• Why Lightbox is bad, by Gary Simon
• The Trouble with Lightbox (and its Variants), by Jonathan Christopher
• Building a Better Lightbox, by Josh Habdas