Maybe because I'm not a native english speaker, but to me 'digitising' means converting an analog medium to a digital one. Like with vinyl, VHS, etc. And 'ripping' is about an extraction process. Like overcoming a protection or more difficult to access mediums like Audio CDs, which don't have a real filesystem, and into a easily accessible single file on a harddrive.

DVDs are already digital, and if they don't have a copy protection, which you have to rip through, you can just copy the files to a hardrive and then, if you want reencode them into one more portable file...