Prior to 1978, U.S. copyright law provided a maximum copyright term of 56 years (an initial 28 years, renewable for another 28 years). After the 1976 Copyright Act (which became effective in 1978), the copyright term was extended to 70 years after the author’s death (or 95 years for corporate works).
The Center for the Study of the Public Domain lists some of the works that would have entered the public domain under the prior copyright law. These include J.R.R. Tolkien’s The Return of the King, Jack Finney’s The Body Snatchers, Alfred Hithcock’s To Catch a Thief, and Disney’s Lady and the Tramp.
Works in the public domain are available for people to re-imagine, recast, and build derivative works. As succinctly put by the CSPD:
The public domain is the realm of material—ideas, images, sounds, discoveries, facts, texts—that is unprotected by intellectual property rights and free for all to use or build upon. Our economy, culture and technology depend on a delicate balance between that which is, and is not, protected by exclusive intellectual property rights. Both the incentives provided by intellectual property and the freedom provided by the public domain are crucial to the balance. But most contemporary attention has gone to the realm of the protected.