Copyright exists to protect the intellectual property and creative work of individuals and groups of people.
For a work to be protected under copyright, it must:
Copyright does not protect ideas or processes, so while no one can hold the copyright for the idea of two lovers from warring families, Shakespeare's Romeo and Juliet itself is a creative work (which has long been in the public domain), and reiterations of that famous work like Romeo + Juliet (1996) starring Claire Danes and Leonardo DiCaprio are protected under copyright. Similarly, no one can hold the copyright for a style of art or a method of painting.
Copyright also does not protect facts. A researcher may spend their whole life uncovering knowledge about some phenomenon in nature; no one holds the copyright to that knowledge. However, not being protected by copyright does not exclude the principles of academic honesty and plagiarism, so while it is not a violation of copyright to report facts, failing to attribute information to its source still constitutes an ethical violation and plagiarism.
Using Copyrighted Works
Using copyrighted works in your own work normally requires the permission of the copyright holder. When a film uses previously recorded music as part of the soundtrack, the filmmakers will have received permission from the copyright holders of the music (and likely paid a fee for usage rights). When an advertising agency uses protected stock imagery, they too should have received permission to use that image. Generally speaking, anytime you wish to use someone else's creative work in your own work, you need to seek permission.
However, there are some exceptions to this rule. Some formerly copyrighted works are in the public domain, meaning they are no longer subject to protection under copyright. Other works have been granted Creative Commons licenses by their creators. These are still subject to copyright laws, but the licenses clearly express what you can and cannot do with the works. There is also the doctrine of fair use, which can sometimes allow for the use of copyrighted material for scholarly, educational, and transformative purposes. The rest of this guide will explain Public Domain and Creative Commons licenses, and help you to locate these kinds of resources to use.
The Public Domain refers to works that are not protected under copyright. Some works are placed in the public domain upon creation by the creators, and other authors or creators place their works in the public domain later on. Under current copyright law, all works published before 1923 in the United States, and works published before 1964 whose copyright was not renewed are currently in the public domain.