For those who aren't long-time open source software developers, especially gamers who write mods or addons (see my previous article) I'd like to provide a quick Q&A to answer some of the questions (and misconceptions) I've seen come up recently. I want to emphasize that I am not a lawyer, but these topics are fairly general, and I believe I'm fairly safe in providing a basic intro. You should read the licenses you apply to your software before you do so, of course.
Question 1: "Are open source and public domain the same thing?"
Answer: Not at all. Public domain works are works that have had their copyright expire, revoked or were not eligible for copyright protection in the first place. Open source software, on the other hand is copyrighted and that copyright has one or more owners who retain the rights with respect to that software. Just because you've allowed others to copy your work doesn't meant that you don't retain control of it. For example, you're the only one who can offer that work under other licensing terms or transfer the copyright to another party.
Public domain licensing (see below) is not open source licensing, though it has some properties in common. That's important to remember.
Question 2: "But don't you give up your rights when you license something as open source?"
Answer: All licenses (regardless of how proprietary they are or are not) represent compromise between the copyright owner and the licensee, but copyright status of a work doesn't change because it was licensed under open source terms. It only means that you've allowed re-distribution and modification of the work.
Question 3: "Can I change the licensing terms of my open source software?"
Answer: You can if you are the sole copyright holder. However, if your software is made up of code that you wrote and code that others wrote, unless they signed ownership of their changes over to you, you do not have sole ownership of the final result, and any change of license must either be allowed by the licensing terms under which they made their changes or must be agreed to by them.
Question 4: "Can someone modify my program / mod /addon and redistribute it without my name"
Obviously, this depends on the terms of the license, but in the case of all of the open source licenses that I know of, no. One exception is public domain software. There's some controversy as to what constitutes public domain software and if you can, in fact, release your own code to the public domain, but as so many developers claim that their code is PD it should be noted that PD software has no copyright, and thus can be claimed by and controlled by anyone.
Question 5: "Why would I want to open source something I worked so hard on"
That's a question no one can fully answer for you, but there are some advantages: it makes it easier and more attractive for others to contribute to your software; if you ever stop maintaining your software, others can pick up the ball; having an open source project under your belt is a nice resume item; and mostly, it just helps to build a sense of community around your software.