ABC Notation  is the text-based music notation system and the de facto standard for folk and traditional music.

In basic form it uses the letter notation with a–g, A–G, and z, to represent the corresponding notes and rests, with other elements used to place added value on these – sharp, flat, raised or lowered octave, the note length, key, and ornamentation. This form of notation began from a combination of Helmholtz pitch notation and using ASCII characters to imitate standard musical notation (bar lines, tempo marks, etc.) that could facilitate the sharing of music online, and also added a new and simple language for software developers, not unlike other notations designed for ease, such as tablature and solfège.

X:1
T:The Legacy Jig
M:6/8
L:1/8
R:jig
K:G
GFG BAB | gfg gab | GFG BAB | d2A AFD |
GFG BAB | gfg gab | age edB |1 dBA AFD :|2 dBA ABd |:
efe edB | dBA ABd | efe edB | gdB ABd |
efe edB | d2d def | gfe edB |1 dBA ABd :|2 dBA AFD |]

Lines in the first part of the tune notation, beginning with a letter followed by a colon, indicate various aspects of the tune such as the index, when there is more than one tune in a file (X:), the title (T:), the time signature (M:), the default note length (L:), the type of tune (R:) and the key (K:). Lines following the key designation represent the tune. This example can be translated into traditional music notation using one of the ABC conversion tools.

The Wikipedia page has lots of links to more information and history.

Learn ABC  page contains several links to resources which will help you learn abc. If you are completely new to abc you may be better to start with the how to … articles or the tutorials – if you are more familiar with abc then the abc standard will give you all the rules.

Michael Eskin from San Diego has written extensive ABC Transcription Tools. His YouTube Channel includes Demo Videos and Tutorials.