This is a wiki (a website that anyone can edit) with sheet music for various songs and other musical pieces.
The sheet music is written in abc, a musical notation language.
There are different sections:
The wiki uses DokuWiki and the ABC2 Plugin.
You can use the abc syntax when it's surrounded by
You can transpose each piece by adding the number of semitones, e.g.
<abc 7 -5>.
The rest uses wiki syntax.
You can try everything out in the playground.