What is the difference between plagiarism and copyright infringement?
Answer
Plagiarism is an ethical offense, relating to taking credit for ideas or expressions that you did not create. We avoid plagiarism by giving credit, using whatever standards and style manuals that discipline uses. For instance, in psychology, most publications rely on the APA Style Manual from the American Psychological Association.
Copyright infringement is a legal matter, and is defined by the laws of the appropriate country -- usually the country the alleged infringer is in. In most countries, copyright infringement requires reproduction (copying) or distributing copies of a copyrighted work, or portion of a work, without permission of the rightsholder. Some other actions -- public performances or displays, or making derivative works such as translations or adaptations -- may also infringe a copyright.
The two categories can overlap, but not all plagiarisms are copyright infringements, and not all copyright infringements are plagiarism. For instance, if you write a paper and discuss a particular scholarly model as if it is your own, and do not cite the original creator, that could easily be plagiarism. However, it would not usually be copyright infringement, since copyright does not cover ideas.
By contrast, suppose you make a copy of an entire book or movie and sell the copy. Giving credit -- listing the authors or creators -- means you have not plagiarized anybody. Unless you have a good reason that qualifies as a defense, this use is likely a copyright infringement, because it is a reproduction and a distribution of a work, without permission of the rightsholder.