The Digital Millennium Copyright Act is better known in legal circles as the DMCA. It is a copyright law in the United States that criminalizes the production and dissemination of any copyrighted technology, devices, or services which are intended to help pirate copyrighted works, whether it is a move, a book, or a website. Additionally, the law criminalizes the bypassing or circumventing or an access control, regardless of whether copyright infringement actually occurs afterward.
The Digital Millennium Copyright Act passed the United States Senate with a unanimous vote on October 12th, 1998. It was signed into law by then-President Bill Clinton on October 28th, 1998. Essentially, the law was passed without much debate because it was considered a common sense solution to the problem of copyright infringement online.
Copyrights are important, they protect the intellectual property of creative individuals in the same way that private property laws protect the physical property of others. The problem, of course, is that, like any law, the DMCA not only has exceptions and loopholes, but it is also occasionally used to harass others—we'll touch on that later.
What is most commonly known about the DMCA is the use of DMCA Takedown Notices. Individuals who run websites and blogs are used to seeing these because they are used quite often in order to notify the website owner of a copyright violation. If the website or blog owner immediately removes the offending material, then they are not legally liable for hosting the material to begin with. This is done in order to prevent 3rd party hosting sites from being liable for the actions of commenters and users who cannot all be moderated simultaneously. However, it has led to some misuse of the Takedown Notice, with many people issuing them without really having a copyright issue at hand. Websites owners do not usually want to be legally liable for anything, so they simply remove the material instead of taking it to court or questioning its veracity.
A number of high profile court cases have involved DMCA-related issues. The biggest was Viacom's lawsuit against Youtube and Google seeking $1 billion in damages. The judge ruled in Youtube's favor. Smaller, less expensive, lawsuits occur everyday regarding the DMCA and copyright issues. Most lawsuits are avoided through Takedown Notices or out of Court agreements and settlements, but there are still DMCA-related lawsuits coming through every level of the court system.
Overall, the DMCA is a law that was much needed and has served the Internet well. Every law has a possibility of being misused. DMCA Takedown Notices are something to pay attention to. If you get one, take a look at it and perhaps have a lawyer look over it. If the lawyer finds there is something to it, you should consider taking the legal threat seriously and removing the material. If not, then you should consider the material on its own merits and obey whatever your own heart and mind tell you. If someone has placed your copyrighted material on a website or blog, then you ought to use a Takedown Notice to make them aware of it and ask, politely, to have it removed as soon as possible.
This is the Wikipedia entry for the DMCA, which is pretty exhaustive.
This is the actual text of the Act, if anyone wants to read it.