Earlier today I needed to grab a couple of shots from a DVD for work, from a presentation we were sent by a client, although there are some software packages for this, you can also easily do this from the terminal.
Open the terminal and type:- screencapture -i ~/Desktop/dvd.png
You should have some crosshairs, now press the spacebar and they change to a camera.
Position the camera over the DVD window and click and you should have a file on your desktop called DVD.png which is a grab of whatever was currently running in the DVD player.
If you plan on making multiple grabs, dont forget to rename the files as you save them or change the name in the terminal prompt to something else each time, e.g. dvd1, dvd2 etc.
Source for this and credit goes to Random Tech and this article http://highschoolblows.blogspot.com/2005/11/take-screenshot-of-dvd-player-in-os-x.html