How is copyright related to plagiarism?
Plagiarism is best defined as the unacknowledged use of another person’s work. It is an ethical issue involving a claim of credit for work that the claimant did not create. One can plagiarize someone else’s work regardless of the copyright status of that work. For example, it is nonetheless plagiarism to copy from a book or article that is too old to still be under copyright. It is also plagiarism to use data taken from an unacknowledged source, even though factual material like data may not be protected by copyright. Plagiarism, however, is easily cured – proper citation to the original source of the material.
Copyright infringement, on the other hand, is the unauthorized use of another’s work. This is a legal issue that depends on whether or not the work is protected by copyright in the first place, as well as on specifics like how much is used and the purpose of the use. If one copies too much of a protected work, or copies for an unauthorized purpose, simply acknowledging the original source will not solve the problem. Only by seeking prior permission from the copyright holder does one avoid the risk of an infringement charge.
Source: "Copyright and Plagiarism," Duke ScholarWorks