There is a difference between the ethical offense that is plagiarism, and the legal offense that is copyright infringement. Copyright is a legal structure that specifies the rights an author has over original works of authorship, such as the right to make copies, make derivative works, and display or perform a work publicly. So, if you write a novel, you can sell copies of it, and license the right to make a movie version of it to someone else, and write a sequel to it using the same characters, but others cannot do so without your permission. If you buy a copy of a copyrighted movie, you do not own copyright on it; rather, you own a license that allows you to view it privately, but does not allow you to do things like show it publicly. (Places like movie theaters that do have this capability pay a separate license to do so, which costs much more than the cost of simply buying a copy of the movie).
Plagiarism does not always equal copyright infringement. If, for instance, a friend gives you permission to copy his or her paper and turn it in as your own, you are not breaking copyright law, but you are committing plagiarism. However, if you copy a paper from the internet and claim that you wrote it, you are both plagiarizing and (most likely) infringing copyright. The only way that this wouldn’t be copyright infringement would be if the owner of the paper had given permission, or had officially placed the work in the “public domain,” which is a legal term signifying that the owner has given up all copyright claims on an original work, and therefore anyone is free to use it however they want. Regardless of whether it is legal to own or reproduce a text you find, it is still plagiarism to submit it as your own work.
Copyright law makes a distinction between an idea and the expression of that idea. Copyright protection only covers the expression of the idea, not the idea itself. Plagiarism, however, can refer to the unacknowledged reproduction of an idea. So, it is possible to plagiarize an idea even if you avoid infringing copyright by changing how that idea is expressed.
Fair use complicates copyright somewhat. Fair use describes situations in which you can use copyrighted material without the author’s permission. It specifies certain kinds of uses that are protected because they have a “transformative” purpose, such as to comment on, criticize, or parody a copyrighted work. In fact, your ability to quote short passages from other scholarly works in your own research is made possible by fair use law. Fair use is thus incredibly important to the flow of information in academic research.
If a copyright claim is made (someone thinks your use of their work does not fall under “fair use”), a judge can use the following factors to determine whether your use of copyrighted materials is “fair”:
This determination can be complicated, but in general, if your use is educational/non-profit, only uses a small portion of the overall work, and is unlikely to affect the market for the original work, the use is more likely to be considered fair (which means you can continue to use the copyrighted material without permission).
Nowadays, we also have legal structures known as creative commons (CC) licenses that allow the original author to retain copyright, but also allow them to specify certain kinds of uses that are legally allowed. For instance, I could apply a CC license to a music recording I made that allows others to download and play my work publicly as long as they give me credit for having recorded the song, but which does not allow them to edit the recording in any way. Alternatively, I could apply a CC license that allows others to edit my original song, as long as they then apply a similar CC license to it so that others may edit that person’s work. CC licenses allow authors to pick and choose the rights they want to give to others in a standardized way so that people don’t have to guess or waste time seeking permissions for each and every use of copyrighted material.