Packetized Elementary Stream
From Just Solve the File Format Problem
A Packetized Elementary Stream (PES) is the basic format by which streams of MPEG-1 or MPEG-2 audio or video, and other content such as subtitles, are stored within certain formats such as MPEG Program Stream.
A DVD has its content in a whole series of these. The VobSub program grabs the PES packets on a DVD that represent subtitles.
