Unfortunately the iOS system is not easily setup to do this, as they want you to buy content off of the iTunes store. Also, the big studios don't want people copying the DVD's and then sharing so there is not a straightforward process for this.

Now, with that being said it is possible with the correct type of software.

Here are a few links to check out:  

http://www.howtogeek.com/howto/31491/convert-dvds-isos-and-avi-video-files-to-mp4h.264-to-play-on-your-ios-device/

http://www.macworld.com/article/153361/2010/08/optimize_idevice_video.html

http://maketecheasier.com/get-dvds-onto-ios-devices/2011/02/14