Digital Millennium Copyright Act

The DMCA passed into US law in 1998 and set the bargain the whole platform runs on. A host that removes allegedly infringing material promptly on notice keeps its safe harbour. It cannot be sued for what its users uploaded. Rights holders get a standard form for demanding removal, and uploaders get a counter-notification that puts the material back after a waiting period unless the claimant goes to court.

Both forms carry a penalty-of-perjury declaration, which is worth reading before firing one off in temper. A DMCA takedown is also a separate mechanism from a Content ID claim: Content ID is YouTube's own private matching system and sits outside the statute entirely.