It seems more and more common that editors are being asked to use content from a DVD- perhaps the client has provided you with last years annual report DVD… and wants you to redit it. (Remember, ripping off copyrighted content is a Federal offense with imprisonment and large fines possible!)
You can use MacTheRipper to get the content from a DVD on your hard drive (OS X only)
MPEG Streamclip is an application that converts MPEG files (including transport streams) into muxed, demuxed, QuickTime, AVI and DV files. http://www.squared5.com/
Here is a tutorial on extracting content with MPEG Streamclip, but it specifies DV codec, which you will in most cases want to avoid – use Pro Res, PJPEG or other higher quality codec instead.