There are many programs available to rip DVDs if you do a Google search. There are many threads here as well that discuss it. The best free method is
HandBrake. Visit their site for the software and methods of breaking the copy protection on the DVDs for your computer's OS to copy them to your iTunes Library then to the iPad.
Ripping the DVDs is not illegal, breaking it's copy protection is in most countries. I wouldn't worry about it if you are doing it for your own use, just don't start uploading them through bit-torrent and pirate sites.