Understanding and recognizing copyright is a balancing act. You must balance the rights of the authors with your rights to use their work, without seeking permission or paying royalties. Under copyright, authors have the right to control the use of their work subject to exceptions permitted under the law. If use of the author's work goes beyond what the law allows you are infringing on someone's copyright.
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 context, downloading or uploading substantial parts of a copyrighted work without authority constitutes an infringement.
Penalties for copyright infringement include civil and criminal penalties. In general, anyone found liable for civil copyright infringement may be ordered to pay either actual damages or "statutory" damages affixed at not less than $750 and not more than $30,000 per work infringed. For "willful" infringement, a court may award up to $150,000 per work infringed. A court can, in its discretion, also assess costs and attorneys' fees. For details, see Title 17, United States Code, Sections 504, 505.
Willful copyright infringement can also result in criminal penalties, including imprisonment of up to five years and fines of up to $250,000 per offense.
For more information, please see the Web site of the U.S. Copyright Office at www.copyright.gov, especially their FAQ's at www.copyright.gov/help/faq.
Copyright is a federal law that protects original works from being copied and distributed without the author's permission unless one of the exceptions applies.
Plagiarism is passing off someone else's work as one's own or lack of attribution. There is no federal or state plagiarism law but there can certainly be severe repercussions for plagiarizing (refer to the Code of Student Conduct).
Copyright infringement example: Incorporating an entire poem by Maya Angelou into a published work without her permission. The poem is property attributed to Ms. Angelou.
Plagiarism example: Using a line or even an entire poem by Maya Angelou in a paper and not attributing the poem to the author or citing the source. It would appear that the poem is the creation of the person writing the paper and not Maya Angelou.