One complicated branch of the American legal system is copyright law. Figuring out exactly what types of intellectual properties you can protect with a copyright, and what can and cannot be taken and used by someone else, is big business. And the subject is getting even more complex.
You've probably seen the little c symbol wrapped in a circle – © – on various documents, artwork, photos – even musical scores. It means that someone has formally registered his or her copyright, which is one's claim to having created the work. The copyright-holder asserts the exclusive right to distribute, sell, make copies of, or perform this work.
Technically you don't need the copyright symbol. Your original work is copyrighted the moment it's finished in permanent form. But to have a prayer of preventing others from, in effect, stealing your work, you must formally register it at the U.S. Library of Congress.
That little c and all it stands for were hard enough for the average person to understand. Now along comes a whole array of new terms and symbols.
One is not a single c but two: cc. The letters stand for creative commons, which is a free license, registered through a San Francisco nonprofit corporation. Creative commons usually means that the author grants permission to others to copy the work at no charge. Sometimes there's a stipulation that the author be credited, or that it not be used for commercial purposes.
That's the meaning of another new copyright symbol: a dollar sign with a slash through it.
And there's one more mark that researchers love. It's the letters PD, and it means that the creator puts the work entirely in the public domain. Anyone, anywhere, is free to use it in any way.
As you might guess, there aren't as many PDs as there are c's and cc's around!