LilyPond is a music notation program. It does not play music, it takes the music you have composed and outputs a score in a beautiful classical engraved style. The music is written in a straightforward ASCII format, no fancy editors are needed and keyboard input is far faster than any mouse driven editing once you pick up the basics.
The main use of LilyPond is to produce finished scores, but it can also produce fragments of music suitable for embedding in word processing or web documents. LilyPond can work on batches of files, and some universities use this method to amass collection of music in their libraries. Speaking of collections, the Mutopia Project
is a large collection of public domain music, which includes much of the music from composers who died more the seventy years ago. It is rather like a Project Gutenberg for music and all of the music is provided as LilyPond files.
LilyPond comes with a 300 page manual and over 400 example files, which should provides all the help you need to use it.