As mentioned in other answers you can use Disk Utility or
dd to create an ISO image of the original disc. But if the disc is copy protected, it contains decryption keys in the lead-in area of the disc which cannot be read directly, and are not part of the ISO image. So if you burn a new disc with this image it will not play on a standard DVD player. Nevertheless, you can play it using a program like VLC which doesn't need the keys, since it is able to circumvent the encryption.
If you want a program that will copy the disc to the hard drive and also remove the copy protection so that you can burn it to a new unprotected disc, MacTheRipper will do that, but it looks like it hasn't been updated in a while. (Only a PowerPC version is listed.)
For storing on your hard drive you might find it more useful to transcode the content to unencrypted H.264 using Handbrake. This will save you a lot of disk space compared to storing the MPEG-2 content that is used on DVD-Video discs. However, it will not preserve the DVD menus, and if you want to burn a DVD that you can play on a standard DVD player then you would have to convert it back to MPEG-2.