There are several different kinds of MPEG streams/files - what's the difference ?
'Normal' MPEG streams (which usually have a .mpg file extension) are 'multiplexed' files with a video stream and (at least one) audio stream. These also contain Presentation Time Stamp information (PTS) so the decoder can sync the video and audio together correctly on playback.
For MPEG-2 these are either 'Program' streams (the common variety) or 'Transport' streams (used for digital TV broadcasting mainly). The MPEG-1 equivalent is a 'System' stream.
'Elementary' streams are video or audio only.
How do I convert between 'multiplexed' and 'elementary' streams?
If you take a Program, Transport or System stream file and run it through a 'demultiplexing' tool it will generate an elementary video file and an elementary audio file by pulling the stream apart. You can reverse the process using a 'multiplexing' tool. TMPGEnc ( http://www.tmpgenc.net ) has these tools - also try looking in the download section of http://www.doom9.org for other tools. Multiplexing and demultiplexing are fast operations since no re-encoding is involved - the data is just being repackaged and headers added or modified.
What are elementary streams used for?
High-end DVD authoring tools quite often will only accept elementary stream files. This is partly because creating the .vob files for the final DVD is basically a specialised multiplexing operation, so it's easier if the source files are in elementary stream format to start with.
Tony