Search extras allow applications to integrate and add value to the search experience in Windows Phone. For example, after and finding a movie, search extras can launch the iMDB application to see reviews, buy tickets, etc.)
In this episode of the Inside Windows Phone show, Jared Brown ( Program Manager responsible for Search extras and Bing integration on WIndows Phone) introduces the feature end-to-end, first with some demos and then with a walk through on how an application integrates with extras.
MSDN documentation for search extras (has the doc mentioned in the video)
Code sample for a search extra - you can use this in emulator. Do a search for electronic (e.g. HDTV 52 inch).
Jaime Rodriguez's blog