The Digital Millennium Copyright Act (Copyright Office Summary [PDF]) was passed and signed into law in 1998.
There are two major provisions:
1. The DMCA makes illegal the "circumventing" of digital rights software (that is, the program on every DVD, CD or ebook that is supposed to prevent it from being copied) for personal gain. However, while you are legally able to make a copy of an item you purchase (in case the original gets damaged, for example), you cannot legally own or create the means of making such a copy.
Law enforcement and government, nonprofit libraries, archives, and educational institutions.
Every 3 years, the DMCA enables the Librarian of Congress to decide who else can be included as exempt. The Electronic Frontier Foundation explains what past rulemaking periods have achieved.
2. In the event of an illegal copy being transmitted online, the DMCA provides "safe harbor" for internet service providers (ISPs, which can include educational institutions if they provide internet access to their students) from copyright infringement claims if they give notice and take down the allegedly illegal item. Because of this provision, ISPs are not liable for copyright infringement; only the person doing the transmitting is.
For more detail, see Chilling Effects' FAQ about DMCA Safe Harbor.
The information presented here is for informational purposes only and should not be considered legal advice.