From DVD Demystified
http://www.dvddemystified.com/dvdfaq.html
DVD-V content is broken into titles (movies or albums), and parts of titles (chapters or songs). Titles are made up of cells grouped into programs and linked together by one or more program chains (PGC). A PGC can be one of three types: sequential play, random play (may repeat), or shuffle play (random order but no repeats). Individual cells may be used by more than one PGC, which is how parental management and seamless branching are accomplished: different PGCs define different sequences through mostly the same material.