Mroogle will have answered that question for you many and plenty of times.
There is MacTheRipper, RipIt and Fairmount to rip (copy the DVD to your HDD while removing the copyright protection) the video DVD to your HDD.
Then there is Handbrake to convert the ripped DVD to a file like .mkv, .mp4 and .avi with MPEG-4 codecs like Xvid and H264.