Okay Amar, here goes.
Firstly, reality: Store-bought DVDs are generally dual-layer 9GB (ish) disks, chocfull of ‘special features’ (mostly pants). They’re also region-coded and encrypted (so the cartels can make mucho dinero).
Now, DVD burners fitted in PCs are single-layer 4GB (ish) devices (as are the blank disks you need to go in 'em).
More reality: You can’t fit 9GB of stuff onto a 4GB disk. So it’s physically impossible to do a straight “disk to disk” copy of a store-bought DVD onto a blank DVD.
Most software that claims to be able to do this actually doesn’t.
In order to make a “backup” copy of a movie from a store-bought DVD you need to do a number of things:
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Unencrypt and copy the raw DVD movie files (VOBS, they’re called) to your hard drive.
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Compress the raw VOBS so that they’ll fit on a 4GB blank disk. (this results in loss of video quality, but it still looks good).
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Burn the now-compressed VOBS onto a blank disk using software that complies with the DVD standards (which effectively creates a ‘proper’ DVD playable in standalone players).
More reality:
In just the way that the movie cartels came up with sh*te like Macrovision copy protection and Region Coding to screw the likes of us all out of as much money as possible, so the DVD burner manufacturers came up with a Cunning Plan…
Some burners will burn some disks but not others. Some players will play some disks but not others…
Blank DVDs come in two main flavours: DVD +R and DVD -R. Then there are the re-writable disks, +RW and -RW.
Some burners will only burn to +R, some will only burn to -R. Some will burn both. Likewise players.
There’s more. Burner manufacturers do deals with blank disk manufacturers. Presumably, if the disk manufacturer coughs up the dough, then the burner will permit his disk to be burned. If not, it won’t burn.
Which means, my swiss doodster, that although you might have a burner that burns +R disks, you could buy a shedload of +R blank disks and find that they are “not compatible” with your burner, because the disk mfr didn’t pay his protection money to the burner maker…
So, even if you use a program like DVDDecrypter to ‘rip’ the movie VOBS off your store-bought CD to your hard drive, and even if you use a program like DVD2ONE to recompress the VOBS to fit onto a blank 4GB disk, and then use Nero 6 to burn a new DVD-compliant disk, there are a host of things which might p*ss you right off.
Oh, and things like subtitles? They’re in a different stream on the DVD. So if you were to say, use such programs as those mentioned about to make a backup copy of say, LOTR special extended edition, you’ll find it really annoying when Arwen and Aragorn start speaking Elvish and the bl**dy subtitles don’t appear. Hypothetically speaking of course.
The other thing you have to be really careful of is to select the correct audio stream too… otherwise the ‘backup’ you make may have the excruciatingly irritating “director’s commentary” bollocks for a sound-track instead of the actual movie 
None of which of course has never happened to me. I’m speaking hypothetically using my DV expertise and extrapolating from… stuff I read somewhere that happened to someone else…