In the United States, copyright is explicitly mentioned in our Constitution:
"The Congress shall have 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."
This language gets at a fundamental idea that copyright should balance the rights of creators or authors with the public good.
Today, U.S. Copyright Law grants creators broad rights to their copyrighted works, including:
What Does Copyright Cover?
Short answer: a lot. Copyright law defines the scope of copyright broadly and automatically assigns copyright to works that demonstrate even a bare minimum of creativity.
Copyrighted works include books, articles, music, films, images, software--the list goes on and on. Yesterday's Google Doodle? Copyrighted. The doodle you made in your notebook during yesterday's class? Also copyrighted.
What Does Copyright Exclude?
U.S. Copyright Law only applies to something that is fixed, tangible, or permanent enough that others can perceive it. It does not apply to:
Other works may also be in the "public domain," meaning that for one of a variety of reasons, they are not copyrighted and can be reused in many different ways.
Keep in mind that a single item may include some material that is not covered by copyright law and some that is. For instance, a research paper may reference scientific data or facts that cannot be copyrighted, even though the author's analysis of those facts would be protected by copyright.
Even if copyright does not apply, other areas of intellectual property law may. The World Intellectual Property Organization (WIPO) offers some helpful information on patents and trademarks as well as copyright.