There’re plenty of free and open-source software licenses and not all of them are as simple as Do What the Fuck You Want to Public License. And even if you collected all 4 freedoms and 650 pokemons, it isn’t enough.
The main thing a developer has to know about free and open-source licenses – some of them let the code be linked from code in another language, and some of them don’t.
For example, if you use something licensed as GNU GPL. your project has to be under GNU GPL too. In other hand, if you use BSD license, you can sell it to Apple to make OS X from it.
Please, check the license before using something from GitHub or Google Code.
Google Code allows only OSI-Approved licenses. You have to remember this table to be safe using projects from it:
|License||Link with code using a different license||Release changes under another license|
|GNU GPL 3||-||-|
|GNU GPL 2||-||-|
|Microsoft Public License*||+||-|
+ – allowed
— – not allowed
! – limited
* Microsoft Public License isn’t included in official list on Google Code, but is approved by OSI too and is very popular on Codeplex.