Copyright infringement is when somebody copies or displays publicly someone else's copyrighted items without permission. Therefore, the owner's exclusive rights are violated. For this reason, in the United States, copyright infringement is a strict liability crime. For electronic and audio-visual media, unauthorized distribution is referred to as piracy or theft.
Usually, the illegal copying of copyrighted material occurs through organized black market reproduction channels. They rely upon new electronic devices and implements that allow obtaining a perfect and exact copy of the original material; or they even download it over the internet with the only purpose of avoiding paying a purchase price. In case of a successful private lawsuit by the copyright holder, the usual outcome is the imposition of monetary damages against the infringer as well as injunctions against future uses.
In order to avoid infringement claims, the easiest solution is to protect your material with the acquisition of a copyright license; and if you are interested in the distribution of someone else's material, you just need to obtain an explicit contract in which the publisher or owner authorizes the purchase of a lawful copy. Sometimes, even though you did not obtain prior authorization for use of a copyrighted material, you are also protected by doctrines such as fair use or fair dealing, which may also provide broad defenses to infringement claims.