MPG is jsut a shortened extension for MPEG that is correct.
but if i download a MPG file and a MPEG file they will play on different media players.
(actually i havn't yet been able to even get windows file manager to recognise MPEG and each time i try to double click on it, it asks what program i should use for it, and will not remember if i say "use this program for this file type in future")
whenever i download a .MPG file in Mozilla firebird it trys to re-name the file to somthing like.
****.MPG.MPEG (no idea why, stupid if you ask me)
so i need to manually rename all MPEG files as MPG (havn't been bothered enough to actually dig deep into windows to fix the issue yet, i just stoped using firebird in favour of mozilla 1.5)
so it is my experience that yes the contents are the same for each file but the extension makes a difference still.
i am a 3d animator and have many many codecs and many video file formats
MPEG is sofar the only one that has really given me extension issues.
(and codec issues, like needing to install licenced DVD software to watch MPEG's (e.g. for the codecs) )
anyway thats my experience