A copyright violation occurs when a copyrighted work is reproduced, in whole or in part, without the permission of the copyright holder. This can take many forms, from using characters from your favourite TV shows in a fan fiction story to pirating thousands of copies of the Harry Potter films and selling them for profit. U.S. law allows for some reproduction of copyrighted material, but this defense is available only at a judge's discretion.
The principle of retaining the right to dictate how a work is duplicated--in essence the "copy right"--and to receive payment from those who reproduce your work comes from the Copyright Act 1709 (know colloquially as the Statute of Anne). Copyright has since been modified to suit the needs of the modern era and the majority of its principles are enshrined in the Berne Convention, which set international standards on copyright law.
Under the Berne Convention, once a work is created it is immediately subject to copyright by the creator. This is normally demonstrated by the use of the copyright symbol ("©") somewhere on the work, but it is not necessary to do so. Derivative works may also be copyrighted, although this will be nullified if they violate the copyright of the original work's owner.
The effect of violating copyright is to damage the value of the original work to the copyright owner. This usually takes the form of financial damage, as occurs when pirated DVDs are sold; reducing the number of sales the copyright holder can make. However, copyright is not wholly limited to suing for damages from violations, and it is possible to grant injunctions to prevent further reproduction even where there has been no financial loss.
Copyright is often mistaken with trademarks, leading to the common misconception that if a person's copyright is not defended vigorously, they can lose it. However, there is no article in copyright law that allows for a copyrighted work to become "public domain" without the copyright owner's permission or her death (after which the copyright can lapse). Similarly, a name cannot be copyrighted, although a logo associated with that name can, as it is a creative work.
In the United States, it is possible to claim fair use as a defense against claims of copyright violation. However, fair use is a defense only where claims have been brought against a work that is a parody, criticism, news report or educational document produced in a not-for-profit manner. Whether a specific work falls into any of these categories is a matter for the court to decide on the facts of the case in question.