Although it is quite possible to create a DVD where all the video is contained in a single, continuous VOB file, the developers of the DVD Specification recognised that it would be useful to keep certain assets separate - for example footage with different aspect ratios, audio streams, subtitles or languages. Or, indeed simply to distinguish the "main show" from the "bonus features".
For this reason, DVDs can be seperated into Titles and Title Sets.
A Title is really just a convenient number to label a particular PGC on a DVD with. When there are several distinct sections of video on a DVD, it is common to seperate them into different PGCs, each with it's own Title (TT) number. A classic example would be to give the main feature on a "Hollywood" release one TT number, and assign others for the bonus features.
Exactly why this term was chosen by the DVD Forum isn't clear - since individual DVD and CD products are usually referred to by retailers as "Titles" in the same way that books are, it seems quite obvious that it would inevitably cause confusion. One suggestion is that the idea was to be able distinguish between episodes of a TV series, for example, but with hindsight it seems that a better name could have been chosen. To make matters worse, even though the idea that "a Title = a section of video" makes a certain amount of sense, it's quite possible to give a PGC on a DVD a Title (TT) number, even if it doesn't contain any video at all ! ( See Dummy PGCs. )
Where there are several Titles all referencing to one VOB file, they are collectively referred to as a Video Title Set, or VTS. ( See below. )
Many authoring applications create Titles with only one PGC by default. In this case the Title can be labelled "one_sequential". This ensures that information about timecode and chapters are correctly displayed by the player. However a Title may contain more than one PGC. In this case it cannot be one_sequential, but there may be 999 PGCs per VTS as opposed to only 99 one_sequential Titles.
It is possible to add, remove and duplicate Title PGCs using HDAE - see here.
All the Titles on a DVD which use video from a particular VOB file are members of the same Video Title Set (VTS). Often a VTS will contain only one title, though. "Spec" authoring applications like Scenarist, Creator and HDAE give the author direct control over each VTS and it's constituent Titles. Many authoring applications however automatically create a seperate VTS for every sequence of video used. These are referred to by various different names:
So creating a new Track, Movie or whatever will add a new Video Title Set to the final DVD, with all the video concerned contained in a single Title. Some applications allow the creation of "Stories" or "Playlists" which add new Title PGCs to the VTS and specify chapters from the VOB to play in a different order than they are laid out in the original sequence.
It is possible to delete and import a VTS into an existing DVD project with HDAE - see here.