You can learn more from: apple.com
If the DVD is mp4 format.You can directly transfer the DVD to iPod using iTunes.
Open your iTunes, click "File" from the upper left corner, select "Add file to library", browse your computer hard drive and find the DVD, select the songs you prefer, click OK and the video will be put in your iTunes. Then you can just transfer the DVD to your iPod from iTunes.
If the DVD is not mp4 format.You need a convert software to help you convert the DVD to MP4 format.And them transfer the video that be converted to iTunes.
DVD Converters
HandBrake 0.9.2 (handbrake.fr) - DVD to AVI/MPEG-4 converter - incorporates many major and exciting changes-including full support for the AppleTV Take 2 and the latest iPhone/iPod Touch firmware. Mac users take note: this release is only compatible with Mac OS X Leopard, 10.5. (freeware)
FairMount (metakine.com/products/fairmount) - is a tool that allows decryption of DVD content on the fly for a convenient access. FairMount does not perform the actual decryption, it simply forwards the data to VLC Media Player which is used for decryption - thus, VLC Media Player must be installed for FairMount to work (freeware)
See link for tutorial