Authors are copyright holders as soon as their work is "fixed in a tangible medium. Authors additionally have exclusive rights to their works for a specific time period under U.S. copyright law unless the author transfers any rights through a written agreement. Copyright protection covers any work, ranging from literary works to motion pictures and sound recordings and anything in between. Learn more about the U.S. Copyright Code here: 17 U.S.C. § 102.
Authors may choose also their own copyright license through Creative Commons, a nonprofit organization that works to enable sharing and use of creativity and knowledge. The Association of Research Libraries "Author Rights" website includes advice and a list of author rights' websites.
Using a publishing agreement addendum to retain author rights
Most publishers offer authors an agreement that transfers copyright to them, but authors can retain some or all of the rights to their work so they may distribute their work and include in their institution's repository. Many universities provide their faculty with an addendum to allow them to freely use their own work in their classrooms or for research. SPARC, with Creative Commons, also offers an Author Addendum - available for free on their website or to download from link below - as a way to revise or amend publishers' agreements and retain control of your author rights.
These widely regarded websites offer background and guidance about copyright.