As a copyright owner you have the rights to copy, reproduce, publish, publicly perform, record, display, and make new versions of your original work.
A copyright work is original if it is not copied from another, if two individuals completely separately and independently created the same copyrightable work, they would both have a valid copyright in theory, of course for two people to create an identical work independently is unlikely. The key issue in proving copyright infringement is that the infringing party had access to the original copyrighted work. Compare this to the broader scope of patent protection wherein if an infringer makes the same product that is patented, they are infringing even if they completely created the product independently without any knowledge of the patented product.
Definitions
Plagiarist-one who copies works that are in the public domain, (with the copyright currently expired) under their own name and publishes as such. A plagiarist is not normally liable for copyright infringement unless a separate contractual issue precludes it, say for a journalist or author.
Infringement-one who publishes another’s copyrighted work under the copyright owners name without permission when the copyright is unexpired.
Copyright Life
The life of a copyright depends upon who is the creator of the work.
For individual or joint authors, the copyright exists from the time of the works creation to 70 years beyond the last surviving authors life.
Works made for hire, sometimes called corporate authorship, the copyright exists from either 95 years from publication or 120 years from creation, whichever is shorter.
Works published prior to January 1, 1978 have a different set of copyright term rules, that had shorter initial terms of 28 years and that required renewals to maintain the copyright term. Thus, determining whether a pre January 1, 1978 published copyright has entered the public domain requires a search of the copyright office records.
Once the copyright term is extinguished the work has entered into the public domain, anyone is free to use the work without permission. Note that whether a copyright is in the public domain is determined on a country be country basis, meaning that a copyright could be in the public domain in some countries but not others. A search is required to ascertain the copyrights status at any given time.
Requirements for Copyright Infringement
Valid copyright on the work, note that protection for fictional works is high and protection for factually based works is low.
Actual copying of the work, shown by access to the copyrighted work by the infringer and substantial similarity between the two works. Note that you can insert tracers, for example unique errors only you know about into the material to help prove copying.
Improper use of the work
Cause of action brought within 3 years of having knowledge of the infringement.
Specific intent or willfulness is not required, as the infringement can be innocent, wherein the infringer had no knowledge that the copyright existed. However, willful copying can increase the damages available to the plaintiff.
Substantial similarity is the most difficult element to prove and requires drawing the distinction between the expression that is protected and the idea, which is not protected. Also factored in is the amount of copying, if a small portion of the work is literally copied, this would point to infringement, however, if the copying were only in the abstract being close to the idea and not the expression, this would point to no infringement.