Silas G:

The GPL doesn’t take any rights away from you. The entire point to it is that when you write code, you own it. It was designed in response to businesses that thought that if you used their code interpreter or compiler, you owed them money and they owned part of your code.

You retain full rights on any code you wrote. You can include any GPLed software in your code and that will never change. You can edit the GPLed code any way you need to to get it to work properly, provided you let people know what changes you made.

What you can’t do, in a nutshell, is pretend you wrote the GPLed code, or edit it and then pretend the edits are part of the original code, or include GPLed code in your code and then sue someone pretending they violated one of your patents.

You may laugh, but all three of those things have been tried many times before with a "more shots at goal" attitude. The idea is often to influence public perception of open source code just by taking people to court, even if every lawsuit is guaranteed to fail. Nonetheless, the courts have upheld the GPL and justice has prevailed no matter how much money was being thrown around, historically.

If you release your code under the GPL, you are free to get money from selling it. But you need to release a full version of the source code that anyone in the world can successfully build your software with (again, this is ONLY if you choose to release YOUR software under the GPL). They can do anything they want with the source code- including building your program and distributing it free, as long as they give you credit for writing it.

Some open source developers make money from donations and CDs and binaries put together for convenience, and for putting together complete systems, but most open source developers make money doing what their job title implies: developing. The idea is that if you put a worthwhile tool out there for people to use, and people really like your work, they’ll ask you to solve problems that haven’t been solved yet, and will pay you to solve those problems. Writing open source software essentially gets your name out there and lets you build a portfolio at a low cost, and helps you get around takes-money-to-make-money catch-22 situations.

You are not obligated to release something under the GPL just because it relies on GPL code. There are some licenses that are incompatible with the GPL, but the GPL in and of itself doesn’t prevent you from agreeing to anything or licensing anything. So you can use all the GPL software you want, and then write your own code, and sell your own code with any license you want, just as long as you either mirror or link to the original source code you used. Or, at the very least, let people installing your software know it depends on other software.

Jon:

Yes, but if you redistribute the program you have to give the person source code.