The ABC files are a standard developed a few years ago. This ABC homepage has the written standard which explains it all plus a lot of software to handle it for downloading.

The MIDI things you see (bunches of numbers printed in red) are due to the efforts of Mudcat member Alan of Australia (thanks again,Alan). Each of them contains a link to the page where you can download the software for creating them and for converting them back to real MIDI files.

If you're unfamiliar with MIDI and MIDI software, there have been a lot of discussions on it previously - do a forum body search on "MIDI" or "software" and you'll get a good idea.