IFOs File - InFOrmation File
IFO Files give the player important navigational information, like where a chapter starts, where a certain audio or subtitle stream is located, etc. This is the reason why it's only possible to rip certain parts of a movie (like a chapter) with a ripper which can read this files.

BUPs File - BackUP File
BUP files are just backup files off the IFOs.

VOBs File - Video OBjects File
A VOB contains several streams multiplexed together: Video, Audio and Subtitles. Video is MPEG-2, audio can be AC-3, Linear PCM, Mpeg 2 multichannel or MPEG1 layer2 2 channel audio. A VOB can contain one main video stream and several multiangle streams, allowing you to switch (as an example) the perspective during the movie. This feature is mostly used to display storyboards or other extra features during playback. The maximum bitrate of the video stream is 9.8mbit/s. Together, video and audio stream must be below 10mbit/s at any given moment. It's possible to have up to 9 different audio streams and you can usually switch the audio stream during playback (this feature can be disabled during the authoring phase of a DVD). It's also possible to have up to 32 different subtitle streams. Subtitles are 4 color bitmaps which are overlayed over the video stream, they're usually not encoded into the video stream.
All DVD movies are stored in so-called VOB files. Vob files usually contain multiplexed Dolby Digital Audio and MPEG2 video. Vob Files are called as follows: vts_XX_y.vob where XX represents the title and Y the part of the title. There can be 99 titles and 10 parts, although VTS_XX_0.VOB does never contain any video, usually jmenust u or navigational information.

CSS
Content Scrambling System. Prioprietary scrambling system for video DVDs. Designed to stop people from making copies of DVDs, most commercial DVDs are encrypted using CSS. During playback, DVDs are then decrypted on the fly. Only parts of the DVD are encrypted (for instance all IFO and BUP files are not encrypted, and VIDEO_TS.VOB often isn't encrypted either) and the encryption scheme is rather weak and was quickly defeated. If you want to know what CSS does, insert a DVD video disc into your PC, start playing the disc using a software DVD player, then close the player. Now copy a 0.99GB VOB file from the disc to your harddisk and try to play back that VOB file in your software DVD player. You'll see a lot of funny colored blocks all over the picture making the movie unwatchable. But you'll also see parts of the movie (the parts that are not encrypted).

DVD formats - DVD5, DVD9
There are a few replicated DVD formats available. It is important to know which format is the most suitable for your DVD production due to the price differences between them.
DVD5 - This is the most common replicated DVD produced. A DVD5 will hold approx 4.7 GB data or 120 minutes of high quality audio and video. All DVDs contain two substrates that are bonded together. On a DVD5, one substrate contains the data on one layer and the other is a pitless layer without any information. A DVD-R is suitable as a master for replication.
DVD9 - This DVD format is increasing in popularity due to its ability to hold up to 4 hours of audio and video or 8.54 GB of data. It is a dual layer disc. A DLT is the only acceptable master for this type of DVD.