In the U.S., there are four main ways to determine when and how you can use someone else's work.
Works in the "Public Domain" are not protected by copyright law and are free for you to use without permission.*
In the U.S., works can enter the Public Domain in a number of ways, including when protection expires as set by law, and/or if the right's holder explicitly gives the work to the public without copyright protection. In some instances, works are not protected by copyright to begin with and therefore fall into the public domain (this includes things such as facts, U.S. Government publications, short phrases, etc.)
*Even if something is in the Public Domain, you still must cite your sources and/or credit the author
The Fair Use doctrine in U.S. Copyright Law.
Section 107 contains a list of the various purposes for which the reproduction of a particular work may be considered fair, such as criticism, comment, news reporting, teaching, scholarship, and research. Section 107 also sets out four factors to be considered in determining whether or not a particular use is fair:
Creative Commons is a nonprofit organization that works "to increase the amount of creativity (cultural, educational, and scientific content) in “the commons” — the body of work that is available to the public for free and legal sharing, use, repurposing, and remixing." (http://creativecommons.org/about/what-is-cc).
Creative Commons licenses provide a way for people to share their work and make it available for others to build on and reuse.