Licensing Overview and Resources
First, the standard disclaimer: This book is not a lawyer, and this does not constitute legal or financial advice.
Licenses are a critical component of any open source project. The license dictates how the project can be used and contributed to by others, and neglecting to include a license does not mean you are putting your code in the public domain. Paul Rubens explains,
As an independent developer you are under no obligation to use any license at all, but that may have unintended consequences. That's because many people believe that by providing no licensing terms they are releasing it into the public domain. In fact, the reverse turns out to be the case: If you release your code without any license then it is automatically copyrighted. That means that no-one can reproduce, distribute, or create derivative works from it.
Common FOSS Licenses
ChooseALicense.com and the Open Source Initiative are good places to help you choose what license works best for your project.
MIT - "I want it simple and permissive."
The MIT License is a permissive license that is short and to the point. It lets people do anything they want with your code as long as they provide attribution back to you and don't hold you liable.
jQuery and Rails use the MIT License.
Apache - "I'm concerned about patents."
The Apache License is a permissive license similar to the MIT License, but also provides an express grant of patent rights from contributors to users.
Apache, SVN, and NuGet use the Apache License. - "I care about sharing improvements."
The GPL (V2 or V3) is a copyleft license that requires anyone who distributes your code or a derivative work to make the source available under the same terms. V3 is similar to V2, but further restricts use in hardware that forbids software alterations.
Linux, Git, and WordPress use the GPL.
Creative Commons - "I have some creative work that isn't code."
Every license helps creators — we call them licensors if they use our tools — retain copyright while allowing others to copy, distribute, and make some uses of their work — at least non-commercially. Every Creative Commons license also ensures licensors get the credit for their work they deserve.
This book and Wikipedia use Creative Commons.
If you have any questions about which license to pick, there are some mailing lists that may help you with your specific situation: