This article is from the Copyright Law FAQ, by Terry Carroll with numerous contributions by others.
This answer is included in both the Copyright and Patents FAQs.
There are basically five major legal differences between a copyright and
a patent in the United States: subject matter protected, requirement for
protection, when protection begins, duration, and infringement. There's
also a sixth practical one: cost.
Subject matter: A copyright covers "works of authorship," which
essentially means literary, dramatic, and musical works, pictorial,
graphic, and sculptural works, audio-visual works, sound recordings,
pantomimes and choreography. A patent covers an invention, which
essentially means a new and non-obvious useful and functional feature of
a product or process.
Requirement for protection: In order for a work to be copyrighted, it
must be original and fixed in a tangible medium of expression; no
formalities are required (see section 2.3). In order for an invention to
be patented, it must be novel (i.e., new), non-obvious, and useful and a
patent must be issued by the United States Patent and Trademark Office.
Start of protection: Copyright protection begins as soon as a work is
created. Patent protection does not begin until the patent is issued.
Duration: A copyright generally lasts for the life of the author, plus 50
years (see section 2.4). In the U.S., a patent lasts for 17 years from
the date granted (in some nations, particularly Japan and most European
nations, the duration is 20 years, and is measured from date of
Infringement: For a copyright to be infringed, the work itself must have
actually been copied from (either wholly or to create a derivative work),
distributed, performed, or displayed. If a person other than the
copyright owner independently comes up with the same or a similar work,
there is no infringement. In contrast, a patent confers a statutory
monopoly that prevents anyone other than the patent holder from making,
using, or selling the patented invention. This is true even if that
person independently invents the patented invention.
Cost: A copyright is essentially free. Even if you want to register the
copyright, the cost is only $20, and the paperwork is much less
complicated than the 1040A short form for filing your income tax, well
within the capabilities of the person registering the copyright. A
patent, on the other hand, is much more costly; there are fees to the
Patent and Trademark Office, and the patent application process is much
more complex, usually requiring the services of a registered patent agent
(and perhaps a lawyer) to draft and prosecute the application, adding to
Philosophically, you can look at a copyright as protecting the author's
rights that are inherent in the work; in contrast, a patent is a reward
of a statutory monopoly to an inventor in exchange for providing the
details of the invention to the public.