This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
Copy protection (sometimes erroneously referred to as "copyright protection")
is a feature of a product that increases the difficulty of making an
exact duplicate. The goal is not to make it impossible to copy -- generally
speaking, that can't be done -- but rather to discourage "casual copying"
of software and music.
The goal is *not* to conceal information from prying eyes; see section
(3-19) for information on encrypting data on a CD-ROM.
A separate but related issue is "counterfeit protection", where the publisher
wants to make it easy to detect mass-produced duplicates. An example of
this is Microsoft's placement of holograms on the hubs of their CD-ROMs.
There are full CD pressing plants dedicated to creating counterfeit software
(the worst offender being mainland China), so this is a serious concern
for the larger software houses.
Copy protection on CD-ROMs used to be rare, but as the popularity of
CD recorders grew, so did the popularity of copy protection. A large
percentage of games released in the past few years have been protected.
A more recent innovation is copy protection for audio CDs, inspired by
the rise of MP3 trading over the Internet. This is more difficult to do,
because the protection must allow correct behavior on a CD player but
altered playback when being read by a CD-ROM drive. The best that can be
accomplished is to force the user to play the music in an analog format
and then re-digitize it, resulting in an imperfect reproduction.
The article at http://news.cnet.com/news/0-1005-201-7320279-0.html is a
nice introduction to the issues.
Some people have questioned whether copy protection is legal. In some
countries it may not be. In the USA, the law allows "fair use" of
copyrighted material, but does not require that the content provider
make it easy for you to do so. So while making a copy of a song for your
own private use may be legal, there is nothing in the law that requires
the publisher to make the material available in an unprotected format.
Copy protection has been around for many years -- some of the schemes
employed on the Apple II were remarkably elaborate -- and has never been
challenged on legal principle.
See http://overclockers.com/tips907/ for an article about why "fair use"
is a legal right rather than a constitutional right in the USA, and what
that means to you. The article also has some interesting quotes from
the courts regarding the DMCA and DeCSS, notably this one: "We know of no
authority for the proposition that fair use, as protected by the Copyright
Act, much less the Constitution, guarantees copying by the optimum method
or in the identical format of the original." In other words, arguing that
"fair use" means the publisher must allow you to make a perfect digital copy
(as opposed to a lower-quality digital or analog copy) is without merit.
The next sections discuss data and audio individually.