If you want to copy a DVD that you have purchased, you can insert it into your DVD-ROM drive to check its contents. On the disc you will find several folders. Normally you should always see a VIDEO_TS and AUDIO_TS folder. The AUDIO_TS folder is used to store DVD Audio, the VIDEO_TS to store DVD Movies.
Since we will be focusing on a movie DVD, so we’ll open the VIDEO_TS folder. In this folder you will see .VOB .IFO and .BUP files. The VOB file contains the actual movie files. This is a MPEG-2 encoded file. Simplfied is MPEG-2 is a method of compression for movies, just like MP3 files are compress audio. The file contains also audio, normally this should also be a compressed format. Mainly it is an improved form of MP3, named AC-3. Besides that contain the .VOB files the subtitles and multiple camera angles.
The .IFO files are the information files, it can be used by your DVD playing device. It gives the DVD player information on what .VOB files to load and when to start a new chapter. The .BUP files are just a backup of the IFO files and ususlly not being used, they will only be used when an .IFO files become somehow unreadable.