DVD movies use MPEG-2 video codec.
You can create or copy & burn .mv2 files on a computer, but the files used on a DVD disk are stored in .vob format, which cannot be copied DVD-to-DVD.
You can extract the video with a ripper though (some people use this to re-encode the movie in DivX and burn it to a CD-R). But for now you can't burn entire DVD tracks from one DVD-video to a DVD-R.
And if you could, remember that a movie is typically > 5 GB on a DVD (two-layer, and sometimes two-layer and two disks or faces)... and a DVD-R tops at "only" 4.7 GB (one only layer, one only face)... so you would need to rip the movie, be able to find a tool to cut it, and find a tool that can burn .vob files. Way too complicated.
When you compare the price of DVD-R and the price of a commercial DVD movie, I'd say that you'd better buy the original movie, it is less expensive and takes less time !
(but in the future, DVD-R will contain much more GB, and this will be another story)