The Music Library Association maintains an up-to-date website on music copyright for librarians; it is full of information that is helpful to musicians and students as well. Below are links to various sections of the site.
The Digital Millennium Copyright Act, passed on October 12, 1998, makes major changes to U.S. copyright law to address digital rights management.
As you can see, copyright law is very complex and can be confusing.
This page contains information that can help you understand copyright and sort through all that legal-ese.