When an author copies someone else's copyrighted work--without credit or appropriate permission--to be sold and published in their own work. For instance, when someone downloads a copyrighted song without paying for it.
Copyright infringement is the act of exercising, without permission or legal authority, one or more of the exclusive rights granted to the copyright owner under section 106 of the Copyright Act (Title 17 of the United States Code). These rights include the right to reproduce or distribute a copyrighted work. In the file-sharing environment, downloading or uploading substantial parts of a copyrighted work without authority constitutes an infringement.
Section 501 of the copyright law states that “anyone who violates any of the exclusive rights of the copyright owner ...is an infringer of the copyright or right of the author.”
Generally, under the law, one who engages in any of these activities without obtaining the copyright owner's permission may be liable for infringement. Nevertheless, there are several limitations of the exclusive rights of the copyright owner. The copyright law provides exemptions from infringement liability by authorizing certain uses under particularized circumstances. These exemptions are enumerated generally in sections 107-122 of the copyright law, including Fair Use.
U.S. Copyright Office