Copyright is controlled by laws passed by Congress, cases heard through the judicial system as well as international treaties that Congress has entered into with other nations such as the Berne Convention.
These laws also attempt to define copyright infringement. The core concept of infringement is using someone else's protected work without their permission. Oftentimes the word infringement is associated with terms such as piracy, illegal downloading or file sharing, bootlegging or simply theft. That portion of the Copyright Act discussing infringement including civil penalties, damages and potential criminal prosectution can be found here.
A collection of various nations copyright laws can be found at the United Nations UNESCO International Copyright Law Collection.