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At its core, copyright is a way of protecting someone who creates something of artistic or original value (broadly speaking). Ideally, it's a way for that creator to have control over their work and get money from it. This is can be particularly important if you create something intangible, like an idea or song. Most countries abide by the Berne Convention, which basically applies copyright protection the moment a work is created.
Beyond that, countries tend to differ on the specifics of copyright protections. In the U.S., for instance, copyright protection is usually lifted following a certain period of time after a person's death (usually 50-100 years later). There's also what's known as "fair use" - a means of bypassing copyright law if you intend to use a work for certain uses like parody, research, etc.
It's a complex issue and in today's world of digital and web-based creations, the legal code is struggling to keep up. Hopefully this is a helpful starting point for any of you creative types out there.
All the 'Happy Birthday' song copyright claims are invalid, federal judge rules (latimes.com)
"None of the companies that have collected royalties on the "Happy Birthday" song for the past 80 years held a valid copyright claim to one of the most popular songs in history, a federal judge in Los Angeles ruled on Tuesday."
Copyright basics: What is copyright law? (copyright.com)
"Copyright protection exists from the moment a work is created in a fixed, tangible form of expression."
Fair Use (fairuse.stanford.edu)
"Fair use is a copyright principle based on the belief that the public is entitled to freely use portions of copyrighted materials for purposes of commentary and criticism."