The Digital Millennium Copyright Act (DMCA) protects certain online service providers, like WordPress or Tumblr, from liability for acts of copyright infringement carried out by their users. In order to be exempt from liability, however, service providers have to comply with certain conditions – the most important of which is the “DMCA take down notice.”
If a copyright holder finds out that his or her work has been illegally posted online, the holder can send a DMCA take down notice to the service provider, requesting that the content be removed. If the service provider then fails to take action, it could be held liable for copyright infringement.
For example, if you upload your favorite episode of Law & Order onto your WordPress site, NBC can send a DMCA takedown notice to WordPress requesting you remove the episode. If WordPress then fails to take action, it could be on the hook for contributory infringement.