I just was asked by a colleague how to do this so I thought it would make a great blog post:
Assuming you have the rights to the DVD to do this, otherwise this is illegal, here is how to do it:
Download and install handbrake: http://handbrake.fr/
Handbrake allows you to rip the dvd onto your computer so that you can burn the dvd, play the dvd,, put it on your ipad, etc. without the disk. It is just now simple movie file on your computer.
If you want to then put that dvd back onto a dvd disk to be played on a dvd player you will use a program called burn (http://burn-osx.sourceforge.net/Pages/English/home.html) to burn the dvd on a blank dvd.
You can do this with any dvd, even netflix and redbox movies, however, it is illegal unless you have the rights to the disk.