The Digital Millennium Copyright Act (DMCA) was enacted in 1998 in order to update copyright law to deal with issues arising from the growth of the digital age. One aspect of the DMCA is that it created safe harbors for entities that act as online service providers (OSPs) to protect them from copyright infringement lawsuits caused by their users posting infringing content.
Another aspect of the DMCA is that it amended section 108 of copyright law, which allows archives and libraries to make reproductions under certain circumstances. These institutions can now make digital copies for reproduction and replacement.
The DMCA also states that you cannot circumvent a technological measure to reproduce or access a copyrighted work. So activities such as de-encrypting encrypted text or jailbreaking a tablet so that it will work on other carrier's networks are considered illegal activities under the DMCA. There are some exemptions to this provision, such as circumventing a technological measure for reverse engineering purposes or for non-profit educational institutions to determine if they should purchase something.