Copyright is law established in the US Constitution that protects the intellectual property of an author, giving them the exclusive rights to their materials for a set period of time. Works protected through copyright can be "literary, dramatic, musical, or artistic works, such as poetry, novels, movies, songs, computer software, and architecture" ("Copyright").
Copyright protects the author from having their work "reproduced, distributed, performed, publicly displayed, or made into a derivative work" without their expressed permission, with the exclusive right to charge money for the use of their work ("Definitions").
Copyright protection has expanded over time. Under current copyright laws, works are protected for the life of the author plus 70 years.
For more information: please enjoy this video from CGP Gray: Copyright: Forever Less One Day
A work no longer protected by copyright is considered in the public domain, which means that it can be reproduced, distributed, performed, publicly displayed, or made into a derivative work.
You can use this Slider created by the ALA Office for Information Technology Policy to determine if a work is in the public domain.
Here are some additional places to find answers:
World Intellectual Property Organization
Open Washington Open Educational Resources Network
Community College Consortium for Open Educational Resources