The basis of the copyright system in the United States is Article I, Section 8, Clause 8 of the Constitution, which gives Congress the power “To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries.” Essentially, this means that in order to encourage the creation of new works (literature, art, music, etc.), we give creators a limited period of time during which they control how their work is used.
It is useful to think of copyright as a bargain between the public and the creators of new works. Although the public might benefit from having the unrestricted ability to copy, distribute, or perform some new work immediately upon its creation (wouldn't it be great if only one person ever had to buy a copy of Harry Potter, and then she could scan it and give it away to everyone in the country?), this would not provide creators with much reason to create anything (how many books would Steven King write if he couldn't make a living by writing?). Therefore, the two sides come together and make an agreement: creators get the right to protect their works, with the understanding that there are some exceptions and that this protection won't last forever.
United States copyright law protects "original works of authorship" in any medium. This includes:
There are other things that are explicitly not subject to copyright protection:
Some of these may be available for protection as trademarks. For example, the distinctive design of the Coca-Cola bottle is not protected under copyright, but it is protected as a trademark.
There are five basic rights protected by copyright. The owner of the copyright has the exclusive right to:
Copyright holders may also transfer any or all of these rights to another party. For example, writers often transfer their copyright to a publisher as part of a publishing agreement.
Previously, anyone seeking copyright protection for a work had to register it with the proper authority or give a notice of copyright (usually with the © symbol) indicating the year of copyright and the owner. Not anymore. Under current law, copyright applies automatically as soon as a work is fixed in a tangible medium of expression. Even the doodles in your notebook are protected by copyright the instant you make them. It is still possible to register with the US Copyright Office, and owners of registered copyrights have better legal cases to protect their works, but it is not necessary.
Right now, the duration of copyright protection for new works is the life of the author plus 70 years. For works produced for hire (i.e., by a corporation), copyright protection lasts for 120 years after creation or 95 years after publication, whichever is sooner. These durations can change if new laws are passed. This does happen from time to time; the most recent was in 1998, when the Sonny Bono Copyright Term Extension Act passed into law (this law is sometimes called "The Mickey Mouse Protection Act," because it retroactively extended the term of copyright just a few years before the protection on Mickey's first cartoon was set to expire). Once copyright protection expires, a work is said to have entered the public domain.
The public domain consists of everything that is out of copyright or ineligible for copyright protection. Creators may also intentionally dedicate a work to the public domain by declaring a forfeiture of their rights. Anyone can use a public domain work without obtaining permission, but no one can ever own it. However, it is possible for works that are not protected by copyright to be protected by trademarks or other intellectual property laws. Likewise, it is possible to transform or incorporate public domain works into new works which may be eligible for copyright protection.
Let's look at an example. The works of William Shakespeare are all in the public domain. In fact, it is easy to find legitimately posted free collections of his plays online. However, a filmmaker may make a movie version of one of his plays, or a publisher may assemble a collection of his plays with criticism and annotations, and these new works could be protected under copyright even though the text of the play remains in the public domain. (Incidentally, the name "Shakespeare" is also registered as a trademark in the US by a company that sells fishing gear.)
Fair use is a defense that allows certain uses of copyrighted material without permission. These uses include things like criticism, commentary, news reporting, education, research, and parody. The limits of fair use are somewhat vague and are judged on a case-by-case basis. The law sets out four factors to be considered when judging whether a use is fair:
It is important to remember that fair use is a legal defense to be argued in court. Even if you believe a use is fair and state up-front why you think so (in the form of a disclaimer, for example), you could still be sued for infringement and required to prove your case to a judge. Also bear in mind that the four factors are not exhaustive. Other considerations may also come into play, such as whether the copyrighted material was used in good faith or whether the alleged infringer was following the generally recognized best practices of her professional community.
The first-sale doctrine is a rule that says that once someone has legally obtained a legitimate copy of a copyrighted work, she may lend, sell, or give away that copy to others. This provision was important in the growth of the movie rental industry, and it has been a contentious issue in the realm of digital content, where things like books and movies are frequently licensed rather than owned.
The fact that ideas cannot be copyrighted but that tangible expressions of those ideas can be is called the idea/expression dichotomy. While a paper describing an economic theory may be protected by copyright, the theory itself cannot. This means that others are free to describe, critique, extend, or use the theory as they wish without needing permission from the creator.
Certain exemptions apply for specific types of institutions, such as libraries and archives (we are happy about this) and public broadcasters. Federal law also provides for compulsory licenses for certain types of work. The most common situation covered by compulsory licensing is the cover song: an artist that wishes to sell a cover version of a song pays royalties to the owner of the song's copyright, but the copyright owner cannot prevent the cover version from being sold.