Copyright is actually a limited bundle of rights that the government grants to authors of original works such as novels, plays, essays, and movies. For a limited time (currently the life of the author plus 70 years, in most cases), copyright gives the author control over who can copy, distribute, publicly perform or display, or create derivative works (such as sequels or translations) based on their work. Authors (and musicians, screenwriters, and so on) often sell or license this right to larger copyright aggregators, such as publishers, movie studios, and record labels. The purpose of copyright is to encourage the creation and dissemination of new works for the benefit of the public.
Copyright is therefore much broader than the norms against plagiarism. Plagiarism is the presentation of someone else’s work as one’s own; copyright infringement can take place even where the user is honest about the work’s true author. As long as you use proper attribution, plagiarism should not be a worry for you. Copyright is somewhat more complex: unless your use satisfies one of the exceptions or limitations described in the Copyright Act, you cannot use copyright protected material without permission. Fair use is one of the most important limitations to copyright.