First, when the material being ripped is not in the public domain, and the person making the rip does not have the copyright owner's permission, then such ripping may be regarded as copyright infringement. However, some countries either explicitly allow it in certain circumstances, or at least don't forbid it. Some countries also have fair use-type laws which allow unauthorized copies to be made under certain conditions. As mentioned above, circumventing copy protection mechanisms, such as the encryption used on most commercial DVDs, may also be illegal in many countries.
Please make sure that it is legal in your Country before you rip a DVD.