Copyright literally means the right to copy something. Copyright law is the protection for works both published and unpublished, provided by the United States. It provides protection for the author's rights to their work. Copyright law also defines specific ways other people may make use of a copyrighted work without breaking copyright - the author's rights.
The Copyright Law of the United States can be found in Title 17 of the United States Code, Chapters 1-8 & 10-12.
Copyright as defined by the U.S. Copyright Office:
A form of protection provided by the laws of the United States for "original works of authorship", including literary, dramatic, musical, architectural, cartographic, choreographic, pantomimic, pictorial, graphic, sculptural, and audiovisual creations. "Copyright" literally means the right to copy but has come to mean that body of exclusive rights granted by law to copyright owners for protection of their work. Copyright protection does not extend to any idea, procedure, process, system, title, principle, or discovery. Similarly, names, titles, short phrases, slogans, familiar symbols, mere variations of typographic ornamentation, lettering, coloring, and listings of contents or ingredients are not subject to copyright.
Ideas, procedures, processes, systems, methods of operation, concepts, principles, or discoveries are not protected by copyright.
In addition, there are certain types of works that are not protected by copyright:
Works in the public domain are not, or are no longer, protected by copyright. They have become free to use without requiring permission. Generally speaking, works enter the public domain due to their age. Copyright protects works for a set number of years, with the number of years dependent on a few different factors. Cornell University's Copyright Information Center offers a great breakdown of the factors that determine when a work enters the public domain. Copyright Term and the Public Domain in the United States