From
http://www.lbl.gov/Workplace/patent/ch1.htmlThe term "Copyright" describes a bundle of rights granted by the U.S. Constitution to the authors of "original works of authorship" including literary (software is considered a "literary work"), dramatic, musical, artistic, and certain other intellectual works. These rights include the legal right to exclude others, for a designated time, from copying, selling, performing, displaying, or making derivative versions of a work of authorship.
Generally, a copyright will last 50 years beyond the death of the author. For works made for hire, discussed later, the copyright expires 75 years after publication, or 100 years after creation, whichever comes first.