The authority of the United States federal government to govern copyright protection stems from the Copyright ClauseArticle I, Section 8, Clause 8 of the Constitution. This clause grants power to the Congress to "promote the Progress of Science and the useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries."  Using this authority, the government established the first legislation on copyright in 1790. This clause is intended to provide for the compensation of authors and inventors in order to promote the progress of science and the useful arts. The copyright monopoly is not intended to reward authors and inventors for their labor , rather, it is part of a "bargain" to ensure that authors disclose their innovation to the public.
Copyright protection is carefully designed to give some exclusive rights to authors, but also to leave enough information in the public domain so that subsequent authors have raw material to work with. One important Constitutional rule that helps "promote the progress of science and useful arts" is that a copyright owner never owns an idea, only a particular expression.  The underlying idea always remains available for other authors to reuse and express in new ways. Another rule is that only original work may be copyrighted - unoriginal work, such as an alphabetical telephone directory, cannot be copyrighted. 
It is much easier to get a copyright than a patent. For one thing, an administrative application process is not strictly necessary: if a work is sufficiently original to be copyrighted, then it is legally considered to be copyrighted from the moment when the work is first fixed in tangible form. Authors may still wish to pay fees and fill out forms to register their work with the Copyright Office, however, because registration is necessary in order to show ownership of the copyright in an infringement lawsuit.  The second reason why copyrights are easier to get than patents is because a work only has to have a "modicum of originality" to receive a copyright - a much lower standard than the "novelty" and "non-obviousness" required for a patent. However, a work that is not very original may receive only very weak copyright protection - if Author A's work includes non-original expression, and Author B creates a work using the same non-original expression, B probably does not infringe A's copyright.
Copyrights currently last for the length of the author's life plus seventy
years; or, if the creation is a "work made for hire," for ninety-five
years.  When a work is made for hire, the copyright
belongs to the author's employer. Many employment contracts provide that copyrights
in the employee's work belong to the employer; a work can also be considered
"made for hire" regardless of contract language if it was prepared
in the scope of employment. Much commercially produced software is work made
A copyright owner has a number of exclusive rights, significantly including
the right to make both exact copies and "derivative works" based
on the copyrighted work.  The copyright is likely
to be infringed when another person copies all or part of the copyrighted
work. Although most copyright cases involve defendants who attempt to sell
the copies or derivative works, it is also possible to infringe copyright
by making copies that are not sold. 
To win a case for infringement of copyright, a plaintiff must prove that
she owns the copyright, and then must show that the defendant copied her work
- if the defendant came up with the same work independently, there is no infringement.
In addition, even if the defendant did copy, many kinds of copying are not
copyright infringement: depending on the circumstances, it may be legal to
copy a very small amount of a work, or to copy elements of the work that are
not original, or to copy elements of the work for purposes of teaching, news
reporting, and other uses - including reverse engineering.
Materials which may be copyrighted or "protected" under the 1976 Copyright
Act include both published and unpublished works , at the moment of fixation.
 The law protects works of authorship
in eight categories: (1) literary works, (2) musical works (and any accompanying
words), (3) dramatic works (and accompanying music), (4) pantomimes and choreographic
works, (5) pictorial, graphical and sculptural works, (6) motion pictures
and other audiovisual works, (7) sound recordings and (8) architectural works.
Computer code is copyrighted as a "literary work," while a program's screen
displays may sometimes be copyrighted as audiovisual works. When registering a
computer program for copyright, the author should classify the work in whichever
of these two categories predominates. 
Copies Created By Using a Program or by Using the Internet
Under traditional copyright law, making a copy can constitute copyright infringement.
But many standard computer operations - and many uses of the Internet - cause
machines to make ephemeral copies of copyrighted digital content. The law
is gradually adapting to digital technology, but it remains the case that
many normal uses of digital technology might technically be considered copyright
A 1993 case held that by turning on a computer which automatically loaded
a copy of the operating system into RAM, a maintenance engineer violated copyright.
 Congress later amended the law to say that people who legitimately own copies of
computer programs may authorize copying for purposes of maintenance; the statute
also specifies that owners of copies may make additional copies "as an essential
step in the utilization of the computer program" or for archival purposes.
 (It is worth noting that software licensing creates an
odd legal twist here: unlike the purchaser of a copyrighted book, the purchaser
of software may be considered a licensee rather than an owner of the copy, and
the purchaser's rights may be affected by the terms of the license. This raises
a host of legal issues relating to contract law, which will not be addressed in
The normal operations of the Internet potentially raise even greater copyright issues. An influential "White Paper" released by the Clinton Administration's Working Group on Intellectual Property in 1995 argued that infringing copying can occur "when an end-user's computer is employed as a 'dumb' terminal to access a file resident on another computer."  Many commentators strongly disagree with this legal conclusion.