One big difference is that pitch and note length in ABC is specific not relative as in Lilypond. Middle C up to the B above that are Capital letters. An octave above middle C and the notes are in lower case letters.

The L: 1/8 in the header sets the 'standard note as an eighth note. Quarter notes have a 2 after them and half notes have a 4. Notes without a number are all quarter notes.

You can 'declare' a repeat section using |: and :| at either end.

There are several programs where you can input ABC files and experiment. Good luck.