What is LilyPond?

From LilyPond Wiki

LilyPond is a music typesetting program, almost like Finale, Sibelius and Encore. LilyPond works differently from these programs and has some advantages that these applications don't have:

• LilyPond is free. It's not just free because you don't have to pay for it, but is always free to use, modify, and distribute.
• LilyPond is open
  • it doesn't mess around with your computer hardware to make sure you pay the software writers lots of money,
  • it produces sheet music using Portable Document Format (PDF).
  • you type in files for LilyPond to process as plain text, not a closed proprietary format which you can't transfer.
• LilyPond produces output for scores and parts in portable formats, such as PDF, so you can print and view the results of your work easily.
• LilyPond works from plain text files. It doesn't use a proprietary file format which forces you to use a particular application to look at your work.
• LilyPond really produces better quality scores: it aims to provide scores of a quality closer to the work of the old music engraving craftsmen.
• LilyPond is flexible and configurable
  • it runs on a variety of Operating Systems (Windows, Mac OS/X, Linux and other Unix variants)
  • it can be configured for your local conditions and personal preferences (for instance, to use the note names in your local language).
• LilyPond is not WYSIWYG (What You See Is What You Get), it is WYSIWYM (What You See Is What You Mean). You don't only place dots on staves. You place music on staves. LilyPond is really logical, and easy to use.
• There are editors with special features for working with LilyPond.
• Some WYSIWYG editors exist too, although they're not really complete.

The developers of LilyPond have written this essay which discusses how LilyPond attempts to emulate the practices used by the craftsmen who have traditionally engraved music.