According to copyright.gov, copyright is "a form of protection grounded in the U.S. Constitution and granted by law for original works of authorship fixed in a tangible medium of expression. Copyright covers both published and unpublished works."
Article 1 Section 8 Clause 8 of the US Constitution, also known as the copyright clause, grants 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."
Ownership of a copyrighted work includes the right to control the use of that work. Use of such work by others during the term of the copyright requires either permission from the author or reliance on the doctrine of fair use. Failure to do one or the other will expose the user to a claim of copyright infringement for which the law provides remedies including payment of money damages to the copyright owner.
Except in the cases of the copyright exceptions that we will talk about in the next tab, individuals must get permission from a copyright holder to use materials. Stanford University Libraries breaks this process down into 1) determining if you need to obtain permission 2) identify who owns the copyright 3) Identify what rights you need for what you want to do 4) planning ahead because getting these permissions can take months 5) contacting the owner of the copyright and negotiate terms and payment and 6) getting your agreement in writing. More of their resources can be found on their website here.
Copyright permissions can be a pain, and sometimes you can have a hard time even finding out who holds the copyright. That is why the Creative Commons system of standardized copyright licenses is so invaluable. Words and Images explicitly state what someone can and can't do when using materials. The terms of the licenses can even be combined together to allow copyright holders to hold the exact license that they want. For example, the image below is Licensed under the Creative Commons which allows it to be shared on this page.
Copyright protection is outlined In Chapter 1 of Title 17 of the United States Codes as:
(a) Copyright protection subsists, in accordance with this title, in original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device. Works of authorship include the following categories:
(5) pictorial, graphic, and sculptural works;
(6) motion pictures and other audiovisual works;
(7) sound Recordings;
(8) Architectural works