Newbie Question :

I've noticed that DVDDecrypter creates 2 files on my machine when creating a image on my machine. Those files are one .iso file and one .mds file. When loading and burning, what file should I choose ? It seems to me that there's no difference but I just want to be sure.

And what kind of information is contained in the .mds file ?

This is Alcohol file I believe, and is created by DVDDecrypter so that another application that reads mds files can access that iso (I think). Whatever. Anyway, you can disable creation of mds file here:

The MDS file is basically a set of instructions that tells your burner how to burn the DVD, it contains the layer break information for burning DL discs and it is recommended to burn from this file when DL burns are being made...otherwise you can use either file for burning a SL disc. I would also recommend using Imgburn for burning, it is written by LUK who gave us DVDD but is still recieving support.

http://www.imgburn.com

If you're burning a DL disc and loads the .iso file instead of .mds file, will the program (ImgBurn) know where the Layer shift are ? (the .mds lies in the same folder...) or will it calculate the optimal layer break ?. I though the info about the layer shift lied in one of the files contained in the image (guess I was wrong).

If I were burning a DL disc I would use the MDS file and not the ISO image, if burning to SL it doesn't really matter. The latest Imgburn has been updated to create an ISO for burning to DL or building from a VIDEO_TS folder for burning to DL media and lets you select your layer break manually if you so wish to do so, or you can let the program do it for you.

Here are a couple of guides:

Burning a DL ISO:
http://forum.imgburn.com/index.php?showtopic=1777

Burning from video files...handy when ripping in File mode vs. ISO read mode:
http://forum.imgburn.com/index.php?showtopic=1780

you're correct in guessing you were wrong. the image contains only the imag eof the movie...ther'es no information on how to burn. that's why there is a separate mds file.

if you use ImgBurn and load the MDS then you're getting as close to a 1:1 backup as possible (i say close only because it's not a true 1:1 since hte copy protection was removed). if you use ImgBurn and load the ISO then ImgBurn will calculate the layer break on its own. with recent releases of Imgburn though this isn't necessarily a bad thing. It does this quite well.