DVD rippers are software tools that copy the content of a DVD. While the ripper itself isn't illegal, using it to create a copy can be. The key factor is copyright protection. Ripping a DVD you own, without copy protection, for your own backup might be legal, depending on your local laws. This creates a digital file for your personal use, not for sharing.
However, most DVDs have encryption that prevents copying. Bypassing this protection to rip the DVD with a ripper is where legality gets dicey.