What is copyright infringement in the United States?

Basically, copyright infringement occurs when a copyrighted work is reproduced, distributed, performed, publicly displayed, or made into a derivative work without the permission of the copyright owner.

Some examples of copyright infringement are:

Downloading and sharing files of music, videos and games without permission of the copyright owner.
Scanning a photograph that has been published and using it on a website without permission.
Placing the text of books on a web page.
Downloading licensed software without the permission of the copyright holder.
Making a movie file available for download on the internet without permission of the copyright owner.

If you commit one of those acts you may be accused of copyright infringement.

If you provide a service that allows others to commit those acts you may be accused of contributory copyright infringement. The United States Copyright Act does not directly deal with contributory copyright infringement, but the United States Supreme Court has held that a person can be liable for contributory copyright infringement when a person knowingly induces, causes or materially contributes to copyright infringement, by another but who has not committed or participated in the infringing acts him or herself, may be held liable as a contributory infringer if he or she had knowledge, or reason to know, of the infringement. See, e.g., Metro-Goldwyn-Mayer Studios Inc. v. Grokster, Ltd., 545 U.S. 913 (2005); Sony Corp. v. Universal City Studios, Inc., 464 U.S. 417 (1984).

If you are accused of copyright infringement or your believe someone is infringing on your copyright right you should speak to an attorney that specializes in copyright law.