Wikipedia, of course, has basically all the information you would ever need to know. But the hard part is to know where to start. I'd recommend starting off by reading about the Apache License and the GNU GPL, which are two popular sides to the same story, each offering different freedoms to the people associated with the code.
But here it is in a nutshell: Apache License lets anyone do anything with your code, including taking it and using it in a closed source product. It gives whoever is taking the code the freedom to do what they want with it.
The GNU GPL, on the other hand, allows your code only to be used in a project that is also distributed under the GPL. In this case you might write some code and prevent a proprietary company from using your work. Here, you're giving freedom to the code itself that it will always be used for "free" purposes.
