MPEG Elementary Stream
MPEG Elementary Stream (ES) refers to a family of compressed media formats defined by the MPEG-1 and MPEG-2 standards (and maybe other MPEG standards?). An MPEG Elementary Stream can appear on its own as a file format, in which case the file can have only video or only audio. Or, it can be combined with other streams in a container format such as MPEG Program Stream.
Most .mpg files use either MPEG Elementary Stream video format, or MPEG Program Stream format.
The term elementary stream can also refer generically to a single video or audio stream (whether one of the standard MPEG codecs or not), regardless of what layer it is at in the encoding process. So, it might be packetized, and/or part of an MPEG Program Stream.
An MPEG-1/2 video Elementary Stream begins with bytes
00 00 01 B3.
An MPEG-1/2 audio Elementary Stream begins with