The Digital Millennium Copyright Act (DMCA) is a federal law that governs copyright disputes on the internet.
The DMCA is often abused in order to censor somebody else's messages. For example, CBS, Fox News, and the Christian Broadcasting Network have complained to YouTube about campaign ads for John McCain ads which use "brief snippets of news broadcasts and other copyrighted material," resulting in YouTube censoring and taking down the items. "Often, the McCain-Palin campaign's general counsel said, those videos are 'immediately' removed for 10 days, which is a long stretch of time during a presidential campaign." Similarly, the Obama-Biden campaign has been targeted by NBC.
One section of the law sometimes referred to as the DMCA 512 creates a "safe harbor" for media entities such as Twitter and Facebook from legal liability for user-created content.