Copyright is the exclusive legal right of the author of a creative work to determine and decide whether, and under what conditions, the original work may be used by others. In the United States, this right is granted by Title 17 of the US CODE.
Using an author's work or part of an author's work and pretending that it is your own original work is Plagiarism.
If you plagiarize work that has been copyrighted by someone else, the author may sue you for Copyright Infringement and/or you can get in trouble for academic dishonesty.
If you copy your own work, which you have previously published, without acknowledging that the work appeared elsewhere, you are are guilty of Self-Plagiarism or Recycling Fraud.
If you have previously signed over or sold the rights to a work, which you created and then self-plagiarized, you can be sued for copyright infringement for copying your own work.
WARNING: Both plagiarism and copyright are complicated, and both have changed over time. A few years back, it was far easier to plagiarize material from the Internet without getting caught. In the case of copyright, the levels of protection for works and the length of time that a work is protected varies. In addition, the laws that provide copyright protection differ based upon whether a work is written, pictorial, visual, musical, software code, or performance based. No one becomes a copyright guru overnight. Expect to spend some time learning about copyright, and always avoid plagiarizing others' works.