What is common law copyright?

Before Congress amended the Copyright Act in 1976, federal law only protected published works. Unpublished works were protected by state common law (law created by courts, as opposed to state statutes) and such unpublished works were therefore said to be protected by common law copyright.

Common law copyright does not exist anymore because the 1976 Copyright Act not only added protection for unpublished works but acknowledged that state law was now preempted by federal copyright law.