What is copyright infringement?
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 infringement.
Penalties for copyright infringement include civil and criminal penalties. In general, anyone found liable for the civil copy right infringement may be order 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 State Code, Sections 504, 505.
Willful copyright infringement can also result in criminal penalties, including imprisonment of up to five year and fines up to $250,000 per offense.
See WMU-Cooley Law School's policies regarding peer-to-peer file sharing (pdf).