A classical music work is in the public domain if its intellectual property rights have expired or they have been forfeited by the copyright holder. Ideally, copyright law systems would ensure that all works end up under a public domain license after a reasonable amount of time. In practice, and because of huge commercial interests, it takes a very long while for a classical music work to go into public domain, and even then there are numerous exceptions, depending on the nature of the work and the country of origin.
Ascertaining the copyright status of a classical music work is a very complicated process, and you should read this article if you are interested in more precise details. If a work is analyzed and it is indeed in the public domain, then it can be used by anybody for almost any purpose. There are however some limitations remaining, depending on jurisdiction, such as for example moral rights.
Furthermore, if a work is public domain in a country, even if it is the country of origin, than that does not mean that it is public domain in every country in the world. Copyright is national, with a complicated web of treaties and bilateral agreements.
At HDCLASSICALMUSIC.COM, we offer a sizable number of public domain classical music works. Most of these are provided by Musopen, who have recorded and intentionally published the songs under a Public Domain license using funding from their own Kickstarter projects. Their quality is superb (both the audio and the performers’ skill), and you can see the passion for classical music of all the persons involved in the project. We aim to cooperate closely with Musopen, and by promoting their works and organization to help them record most of the great works of classical music.