JAM is both the software and file format for representing music as human-readable and human-writable text.
Unlike the ABC

Abc notation

ABC notation is a shorthand form of musical notation that has been in use since at least the 19th century, In basic form it uses the letters A through G to represent the given notes, with other elements used to place added value on these - sharp, flat, the length of the note, key, ornamentation...

notation, another text-based music format, that is best suitable for one-voice tunes,
JAM is mainly focused on chords.

Here is an example of jam notation:
### LULLABY OF BIRDLAND

Dm7 Bm7-5 | E7 A7 | Dm7 / | Gm7 C7
F+7 F7 | Bb+7 Bbm7 | F+7 / | Em7-5 A7
Dm7 Bm7-5 | E7 A7 | Dm7 / | Gm7 C7
F+7 F7 | Bb+7 Bbm7 | F+7 C7 | F+7 /

Am7-5 D7 | Gm7 / | Gm7-5 C7 | F+7 /
Am7-5 D7 | Gm7 / | Gm7-5 C7 | F+7 A7
Dm7 Bm7-5 | E7 A7 | Dm7 / | Gm7 C7
F+7 F7 | Bb+7 Bbm7 | F+7 C7 | F+7