A format of music interchange which is very popular amongst the Folk community. It is simple, powerful, readable by humans in its raw form, supported by free software and has a massive canon of work. It is primarily of use for writing out a melody line, and therefore is used by many instruments from Tin Whistle to Banjo. It was invented by Chris Walshaw and has gained a large following in the last 15 years.
The ABC home page is a good starting point to the notation, the software available and the tune collections. There are many good programs now which will convert abc to score and even play the tune back. There are versions for PDAs, PCs and Macs as well as services on the web for remote conversion. There are comprehensive notes and a tutorial on the site.
In this example, the abc file is on the left and a section of one of the conversion outputs is on the right, showing the score and Tablature for Mandolin / Tenor Banjo / Fiddle. The lines at the top of the abc are information about the tune. X:21 is simply the tune number (a file can contain more than 1 tune). T: is the title, M: and L: are the Meter and default note length, R: is the type of tune and K: is the Key. The rest is the tune written out as notes (A to G), durations - multiples of the default and Bar lines (|). There are 1st and 2nd repeats at the end of the A and B parts. The notes follow the octave convention (low to high) A, to G, ... A to G ... a to g ... a' to g'. Note the default note length was 1/8 (quaver) so the first 4 notes (d2) mean d as a crotchet.