The VOB is a collection of one or more cells. An entire title could use just one VOB, but they usually use more. Sometimes the use is arbitrary, usually along the lines of a new VOB for each chapter, and within the VOB cells for each scene. This is not a requirement. In fact, there is only one place where seperate VOBs are required, and that is multiple angles.
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. AC3 is pretty much the standard and MPEG-2 multichannel can only be found on very few discs (one example is "In the line of fire, PAL edition") as this format was initially considered to be the standard format in Region2 (Europe and Japan) but was later dropped. PCM is mostly found on music DVDs and MP2 on cheaper productions. PCM is high quality uncompressed audio which takes a lot of space, hence it's not an ideal choice for full length movies with extras and possibly multiple languages. AC3 streams have a bitrate between 192 and 448kbit/s. 192kbit/s is used for 2 channel sound, and 384-448kbit/s for 5.1channel surround.
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.
Related Articles:
What's DVD?
How to rip/convert dvd .vob on mac!
