Copyright laws protect the creators of intellectual property including all forms and expressions of creative works. Creative works include but are not limited to written works, music, software, paintings, movies, and television shows. These rights include protection against unauthorized reproduction, distribution, and performance of protected works.
Copyright Clearance Center - Overview
In the United States, there is in place the fair use doctrine which allows copying and distribution of copyrighted works in specific circumstances. Fair use usually covers areas such as education, research, commentary, and criticism if it is done for non-commercial reasons within reasonable limits.
Works within the public domain are not protected by copyright law. For various reasons, those works may not have ever been under copyright or the protection has expired. Examples of works in the public domain include the Bible, Shakespeare, and Charles Dickens among many others.
There is a growing trend within academia toward open access publishing. There are multiple types of open access licenses, but the focus is on the free access and distribution of works that would otherwise be eligible for traditional copyright coverage. Open access works are typically free to access and share. Different types of licenses may or may not also allow the modification of those works as long as authorship is clearly stated.