Short answers, pulled from the story.
Copyright is a type of intellectual property giving its owner the exclusive legal right to copy, distribute, adapt, display, and perform a creative work, usually for a limited time. Rights include reproduction, distribution, public performance, translation, adaptation, and control over derivative works. The World Intellectual Property Organisation divides these into economic rights and moral rights.
The Statute of Anne, enacted in 1710 in England and Scotland, is widely considered the first legislation to protect copyrights. It followed the Licensing of the Press Act 1662, which had required publications to be registered with the Stationers' Company primarily as a censorship measure. The US Copyright Act of 1790 modeled itself on the Statute of Anne.
The Berne Convention of 1886 first established recognition of authors' rights among sovereign nations, rather than through bilateral agreements only. Under the convention, copyright protection is automatic from the moment a work is fixed in a physical medium and requires no registration. The US did not sign the Berne Convention until 1989.
For most existing works in the US, the term is a fixed number of years after the date of creation or publication. The Copyright Act of 1976 changed the calculation to the life of the author plus 50 years, and in 1998 the Copyright Term Extension Act added 20 more years. Works published before 1929, other than sound recordings, have expired copyrights and are in the public domain; for sound recordings the applicable date is before 1923.
Fair use, codified by the Copyright Act of 1976 at 17 U.S.C. section 107, permits copying and distribution without the copyright holder's permission under certain conditions. Courts weigh four factors: the purpose and character of the use, the nature of the work, how much was taken, and the effect on the market for the original. The US Court of Appeals for the Ninth Circuit held in Lenz v. Universal Music Corp. that a copyright holder must consider fair use before issuing a takedown notice.
The idea-expression dichotomy holds that copyright protects only the original expression of an idea, not the idea itself. This principle was first clarified in the 1879 case of Baker v. Selden and was later codified by the Copyright Act of 1976 at 17 U.S.C. section 102(b). It means, for example, that copyright in a specific cartoon character does not prevent others from creating different works about similar characters in general.