Abc notation is a simple but powerful ASCII musical notation format. Devised by Chris Walshaw, abc is widely used for the notating and distribution of tunes, particularly on the internet. Very popular in traditional music circles, it is also gaining in popularity in early music.
ABC notation is a shorthand form of musical notation. In basic form it uses the letters A through G, letter notation, to represent the given notes, with other elements used to place added value on these - sharp, flat, the length of the note, key, ornamentation.
A good introduction to the subject can be found at http://www.lesession.co.uk/abc/abc_notation.htm with further information on the Wikipedia page .
The home page for detailed information about ABC notation can be found at http://abcnotation.com/