The Digital Millennium Copyright Act (DMCA) criminalizes the distribution of copyright-protected material, and targets music, film, and software piracy in particular. This means that no one can post copyrighted material without the explicit permission of that copyright holder.
Under the DMCA, copyright holders and their agents can demand removal of allegedly infringing content. To do that, they must provide a complete takedown notice (learn more about what that entails here).
This notice, which must be filed by the copyright holder or representative for them, is sent to the service provider’s DMCA agent, which all service providers must appoint and register with the U.S. Copyright Office.
Once the notice has been received, the host has to first make sure it is a complete notice and then they are to either remove or disable access to the infringing work, usually by deleting the content with the infringing work.
With that done, the host then usually contacts the client involved, who in turn has the opportunity to respond.
So, you might be asking, how does a website that is built off the content of others (like YouTube) operate? There are some exceptions to this law. They are as follows :
1. Conduits: Services that were not destinations in and of themselves, (IE: Broadband access providers) were granted complete safe harbor for infringements that passed through their networks.
2. Caching Services: Services that cache data temporarily, such as those used by many broadband providers to speed up access, were also granted complete safe harbor.
3. Web Hosts: Services that host content were given safe harbor provided they had no knowledge of the infringement, lacked the ability to control it, did not encourage it, did not profit directly from it and work to expeditiously remove infringing material after receiving proper notification.
4. Information Location Tools: Similar to Web hosts, information location tools, including search engines and directories, were given safe harbor provided they met a similar set of criteria.
So, for example, if I uploaded a video to YouTube that contained your song, I would be the one responsible for the copyright infringement, not YouTube, as they are just the web host. However, they would assist in the process of a DMCA takedown.
You can learn more about your rights as a copyright holder here.