The idea of copyright did not exist in ancient times, when authors frequently copied other authors at length in works of non-fiction. This practice was useful, and is the only way many authors' works have survived even in part.   Richard Stallman 

Copyright definition

Copyright is the ownership of an intellectual property within the limits prescribed by a particular nation's or international law. In the United States, for example, the copyright law provides that the owner of a property has the exclusive right to print, distribute, display and copy the work, and permission must be obtained by anyone else to reuse the work in these ways. Copyright is provided automatically to the author of any original work covered by the law as soon as the work is created. The author does not have to formally register the work, although registration makes the copyright more visible. (See Circular 66, "Copyright Registration for Online Works," from the U.S Copyright Office.) Copyright extends to unpublished as well as published works. The U.S. law extends copyright for 50 years beyond the life of the author. (see Public domain tab for more info)

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What is covered by copyright?

Works afforded copyright protection include literature, journalistic reports, musical compositions, theatrical scripts, choreography, artistic matter, architectural designs, motion pictures, computer software, multimedia digital creations, and audio and video recordings. Copyright protection encompasses Web page textual content, graphics, design elements, as well as postings on discussion groups. 

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A brief history of copyright laws
How Youtube handles copyright.
Gangnam Style & Harlem Shake: The New Era of Music Copyright