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"Public domain" refers to every creative property in the world that isn't controlled by any one corporation or person, and it includes the Bible, Robin Hood, most novelty sex positions ... it's the central nervous system of our entire popular culture, and whether or not you knew what it was before I just told you about it, you owe a small part of your life to it, because it's been responsible for most of your favorite things.
Do you like TV shows where the lead character is an obnoxious asshole who's smarter than everyone else? Well, House of Cards, Sherlock, Lie to Me, and Suits only got made because of the success of House, M.D., which was a medical reimagining of Sherlock Holmes, a character that they could adapt for free because he's in the public domain. Or maybe you love Christmas, where everything from your Christmas carols to the second best Christmas movie ever made to (as Maxwell Yezpitelok already explained) the popularity of It's a Wonderful Life come straight from public domain. Or perhaps you're into musical theater (don't look at me like that; I just don't want anyone to feel excluded), in which case Les Miserables, The Phantom of the Opera, and Wicked were all adapted from works that anyone could use for free, and might not have ever been created otherwise.
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"OK, party's over, assholes. You're all under arrest."
Hell, even our cultural obsession with zombies is owed to something entering the public domain: George A. Romero forgot to add the copyright claim to the beginning of Night of the Living Dead, and since 1960s copyright law worked the exact same way as calling shotgun, that meant the rights to that movie were shared with everyone. Which is lucky for us, because the unauthorized sequels and remakes ended up creating what you now know of as a zombie movie -- the only reason anyone thinks of zombies as wanting to eat our brains is because of one of these sequels. Romero kinda got screwed, but we all reaped the benefits.
Obviously I'm not saying that everything should be public domain, but since Sir Arthur Conan Doyle isn't going to be collecting royalty checks for Benedict Cumberbatch's Sherlock show (on account of him being super dead), it's clear that everyone wins by having the character end up in public domain eventually. And that's exactly how it used to work: Until recently, your "rights" to a story you created were yours for up to 56 years after you created, at which point it automatically entered the great casserole of human creativity. So at the beginning of every year, that casserole gets a bunch of new ingredients.
"I added zombies because every goddamn thing in the world has zombies now. Enjoy."
Theoretically, anyway. This year, we should have gotten Superman, Atlas Shrugged, and On the Road (among others). Instead, we got ... well, jack shit. And we'll be getting seconds of jack shit every January until 2019, because some assholes from (mainly) the film industry have spent the past few decades continuously lobbying to change the laws and extend copyright protection for as long as possible. They're clinging to their intellectual property like an unsightly blemish on an otherwise majestic set of genitals, and they're just as inhibiting.
And before you say "Well, weren't you just saying that people need to make money off the stuff they create?" keep in mind that the people making money off Snow White and the Seven Dwarfs sure as shit didn't create it. I don't think anyone who worked on that movie is even alive anymore, and yet Disney has kept control because they're Disney, and you're going to lose any argument with them because they own all the words you'd need to use to prove them wrong. The way the law works right now, anything published between 1950 and 1963 will remain under the control of whoever owns the copyright for, in some cases, 95 years after the publication date. For comparison, a patent on a new drug only lasts 20 years after it's invented, so whoever cures cancer will have less control over their contribution to society than the makers of a Larry the Cable Guy movie.
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"Take two of these eatin' pills and git 'er done."
But somehow, it gets even worse, because now companies can legally copyright things they didn't even create. Once you make an adaptation of a work that's in the public domain (like, say, Disney's entire filmography), you gain the rights to all the "copyrightable elements" of that adaptation. What this means is that when Sam Raimi was making Oz the Great and Powerful (based on The Wizard of Oz books by L. Frank Baum, which are in the public domain), he had to be sure that he didn't use any "copyrightable elements" from the movie version of The Wizard of Oz, since that's still owned by Warner Bros. That's probably why, in Oz the Great and Powerful, the witches were zombies, the monkeys were demons, and the movie was un-fucking-watchable.
Look, I'm not ranting like this because I hate capitalism, or even major corporations -- movies like Aliens and Pacific Rim are only made because big corporations can throw money at weird ideas. But I also love creative people, because they make life bearable, and I want them to be free to keep saying weird, funny things about this big stupid world. And if we give the guys in the suits too much control over what we can and can't say or create, that's just not going to happen as much, and I'll have to go back to arson for my emotional release.
JF Sargent has a Twitter, a Facebook, and a free sci-fi novel that you can illegally reproduce in 95 years (and counting).