When a composer writes a piece of music, he owns that music unless he gives ownership away. That means if someone wants the musical score, they must abide by his requirements. Equally, if they wish to perform the work they must give him royalties.
But copyright falls away after a number of years. The composer, his heirs and descendants, don't own it forever. The precise point when copyright vanishes depends on national law. When a work falls out of copyright, it is in the public domain.
Classical music copyright law is fiddly and complex, but the basic rule is that copyright persists for the composer or other author (if any) for 50 years after his death (Canada) or life plus 70 years (EU).
It's in that space where a work enters the public domain that IMSLP operates. Thousands of classical music scores and recordings emerge from copyright every year. IMSLP, run by volunteers using Wiki collaborative software, upload these scores and recordings to a server so they are available for anyone to download.
Like many good ideas, IMSLP is simple and elegant. In 2009, IMSLP won the MERLOT Classics award for Music and was named one of the Top 100 Web Sites by PC Magazine. It gets millions of hits per day.
At the time of writing, IMSLP holds over 71,000 scores by over 4,000 composers. These works would otherwise be sitting in libraries gathering dust, but now can see the light of day. It's a fabulous project. Anyone can get involved.