A problem that faced music lovers at the beginnings of the Information Revolution was how does one transfer or display musical score (or sheet music) electronically in a meaningful way. As computers became more popular as a tool for composing music several programs emerged, each with their own proprietary format. A new program would be developed along with a format that the program could understand, however, only that program could understand it. Because of this, it was impossible for someone to share music in a digital form with someone who used different software. To bridge this gap, MusicXML was created.
Developed by Recordare (reh-co-DAH-ray), MusicXML is an open standard birthed from two existing music formats, MuseData and Humdrum, but incorporates the XML structure. It was designed to act as a universal translator for standard Western musical notation. For those who aren't familiar, XML is a W3C standard for organizing information in computer documents. (Examples of the use of XML are the new MS Office XML and XHTML.) MusicXML is not open source, however it is a royalty-free format, which means people can create programs that use MusicXML without having to pay for its use. Recordare decided to allow this because of the need for a standard, and in the past such standards have been royalty-free formats (such as JPG or MP3).
Because of the open standard and its versatility, groups such as Project Gutenberg are using MusicXML as the format for archiving musical score. Though the format is relatively new, it has caused quite a stir, but has yet to significantly take off.
Here is an example of what MusicXML looks like in plain text (this is only two notes of one measure):
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE score-partwise PUBLIC
"-//Recordare//DTD MusicXML 0.6b Partwise//EN"
"http://www.musicxml.org/dtds/partwise.dtd">
<score-partwise>
<part-list>
<score-part id="P1">
<part-name>Voice</part-name>
</score-part>
</part-list>
<part id="P1">
...
<measure number="1">
<note>
<pitch>
<step>F</step>
<octave>4</octave>
</pitch>
<duration>3</duration>
<type>eighth</type>
<dot/>
<stem>up</stem>
<lyric>
<syllabic>single</syllabic>
<text>du</text>
</lyric>
</note>
<note>
<pitch>
<step>E</step>
<alter>-1</alter>
<octave>4</octave>
</pitch>
<duration>1</duration>
<type>16th</type>
<stem>up</stem>
<lyric>
<syllabic>single</syllabic>
<text>nicht,</text>
</lyric>
</note>
...
</measure>
</part>
</score-partwise>
Sources:
http://www.musicxml.org
http://www.ibiblio.org/gutenberg/music/music_helpex.html