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CopyLeft
The spirit of the GPL is rooted in a concept called CopyLeft, which has an unfortunate and confusing moniker. When you create intellectual property—a book, a song, a computer program—as the author, you automatically have certain rights, such as the right to distribute and control who else can distribute your work. For example, the copyright of this article is controlled by InformIT. You can't copy it and put it on your website without their permission. Software development companies use copyright protections as a way of controlling who can use the code that they've written.
So what's the deal with "CopyLeft"? The FSF's goal was to create a way to protect the freedom of people to distribute source code, without encumbrances. So why not just place software in the public domain, giving up your copyrights? That would work—people would then be free to use your code as they saw fit. However, they would also be allowed, by virtue of the process of changing the code in some manner, to place their own copyright restrictions on the new work. Suppose I wrote a simple word processing program and placed it in the public domain; someone else could come along, add a "boldface" feature, and then they would hold the copyright to the new version. There's nothing inherently wrong with that scenario, but it wasn't the goal of the FSF. Their goal was to create more collaboration and freedom.
The answer was CopyLeft. The idea here is that the author of the copyrighted work gives up all rights to the work, but places on it one restriction: If you use it as the basis for your own work, or redistribute it in any way, you have to provide free access to your source code as well. That doesn't stop you from selling the code, but it does mean that you have to show people the work you've done and the changes you've made from the original work. That's the basis of the GPL.