ASCII is a text format that was originally designed for communications. It has since been re-vamped and found useful for the composition of musical scores.
ASCII only works for instruments that can be written in tab (stringed instruments) and does not accurately describe tone or rhythm for instruments such as Percussion (drums), Wind Instruments, and Piano style instruments.
In other words, It is a simple tab format (.txt) that anyone (regardless of computer hardware capabilities) can use.