How to take a physical dvd movie and put it on a portable hard drive?

This has always baffled me and I cant get a straight answer on how to do it. When people say I have over 200 movies on my hard drive. I think "did they buy them from iTunes?" That would cost a lot, or ill hear people taking dvd movies and transfer them to a hard drive. I hope someone can finally answer this for me. I have a couple road trips coming up and would like a couple movies to watch.