DMCA = Digital Millennium Copyright Act. This law give certain recourse to copyright holders in pursuit of copyright violators. Part of the Act is a provision that a copyright holder can send a "cease and desist" notice to an ISP regarding a customer that the copyright holder believe to be acting in violation of the copyright holders rights.
What isn't clear to me (and perhaps anyone other than an attorney specializing in copyright law) is how the notice should be handled by the ISP. It seems that the law requires the ISP to forward the notice to the putative offending customer; and some ISPs actually do that. -- Norman ~Oh Lord, why have you come ~To Konnyu, with the Lion and the Drum