A modal pop-up is a window created within the page that instantly displays and centers content and establishes focus. Modal pop-ups can be useful for image zooming, watching video or asking for login credentials, like in the example below from TextLinkAds.com.

Other sites like NikonUSA.com are using a modal pop-up in the form of a "lightbox" to show the full-size image when a thumbnail is clicked.

 
This technique makes interaction speedy but may confuse your customers if they don't know how to get out of it. Always create a very clear (and large) close button in the top right of the pop-up.

Modal pop-ups are particularly useful to ensure visibility of an important message, or to prompt the customer for input before continuing. If your site includes circumstances where decisions need to be made, it will probably benefit from these enhancements.