iTunes don't support DVDs so I think you'd have to download a DVD ripping programme somewhere that will convert the DVD into a .mov .mpeg .mp4 and then put it in iTunes and sync your iPad. That could be wrong but it's what I've always thought.
EDIT: Though, these days some dvds/bds are released with digital copies. I believe if you buy a dvd in tescos and have a club card they give you a code to download a digital version (that may have been a limited period offer I don't know).