U.S. copyright law gives a creator exclusive rights to use and reuse their creations and to authorize others to use their creations. Any individual can write text, create artwork or other images, compose and perform music or other sounds, write computer code, or create any other original content. A creator can also be a corporation.
Royalty-free, or RF, refers to copyrighted media or intellectual property that does not require ongoing royalty or licensing fee payments; instead, you may pay a one-time fee in order to use the media multiple times. There may be a limit as to the number of times you can reuse the media. RF licenses are not exclusive, so others may purchase and use the same media as you.
Copyright is automatic but is not the only way to manage original works. Content can be released into the public domain for everyone to use. Works may also be made available under a legal license other than copyright. The Creative Commons organization has written legal licenses that allow reuse of content less restrictive than copyright, for example, specificying that all reuse of content is permitted as long as the original author is credited. These licenses were originally created under U.S. law but are being rewritten in countries all over the world.
Alternatives are sometimes called "copyleft."
Section 107 of the copyright law covers the doctrine of fair use. Fair use permits limited use of copyrighted material without obtaining the copyright holder's permission for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research.
Fair use generally applies to work done for nonprofit educational purposes. However, student websites that are freely available online may not be covered by fair use, especially if the assignment or class the work was done for is over and the website remains online.
These sites and documents will help you understand copyright law in the United States.