Copyright's legal monopoly results in a financial incentive for an author to produce original works. He can earn income in two ways: by licensing the use of his work in exchange for royalties, or by selling his copyright to someone else. Novelists, for example, typically sell their copyrights to publishing companies. Because copyright law allows authors to profit from their work, some are able to devote themselves full-time to creative pursuits. This benefits society as a whole by increasing the number of available creative works.
Copyright's legal monopoly generates a great number of creative works, but places restrictions on their use. The owner of a music CD, for example, is not permitted to burn copies and distribute them to friends, even free of charge. This prohibition more or less restricts the legal use of copyrighted material to those who can afford to pay royalties (typically included in the retail purchase price of a copyrighted work). To mitigate the harshness of this legal monopoly, copyright law allows anyone to use a small piece of a copyrighted work for a socially beneficial purpose such as education. This is known as the "fair use" exception to copyright law.
The Public Domain
Copyright law must balance two competing concerns: it must encourage authors by allowing them to profit from their work, and it must encourage the free flow of ideas. In addition to the fair use exception, copyright law encourages the free flow of ideas by placing expiration dates on copyrights. Although the system is complex, a work created today by an individual author (as opposed to a corporation) will expire 70 years after the author's death, no matter who owns the copyright at that time. After that, the work will enter the public domain. Copyright expiration is why many classic works, such as Victorian era novels, may be freely used by anyone without concern as to whether or not the use is "fair."
Advances in Technology
Advances in technology have challenged the enforcement of copyright laws. Peer-to-peer file sharing, for example, is so widespread that it is virtually impossible to effectively police. The Digital Millennium Copyright Act of 1998 addresses some of these problems. It criminalizes the development and distribution of technology designed to circumvent technological copyright protection measures, such as copy protection for CDs. It also provides a means for interactive websites such as YouTube to avoid liability for infringing material uploaded by users.