To understand the ABC music notation, imagine you're a beginning music student who has pecked out a tune on an instrument and wants to put it on paper. The first thing you might think of is writing down the notes by their letter names:
CCCDEEDEFGThis might be recognized by some people immediately. To make it more recognizable, your second guess might be to show how long the notes are. How would you do this? Well, one way might be to follow each note by its length. This tune is now:
C2C2CDE2EDEFG4Most people will recognize the tune now. The main thing we might do to make it more readable is to break it up with spaces to show the rhythm better. In fact, since our keyboard has a character that looks like a bar line, why not use it? Now the tune is:
| C2C2 CDE2 | EDEF G4 |You now know maybe a quarter of what you need to read and write tunes in ABC. There's a lot more, such as how to indicate octaves. CDEFGAB are the lower octave and cdefgab are the upper octave on the staff, and there's notation for other octaves. There are header lines where you can put information like title, composer, meter and key.
To learn more, read this ABC intro in plain text, or this ABC primer in HTML,