Viral software

When you are in the business of creating software you'll have to know that not all software and business models allow for what you've probably done before you were making money in this field, basically copy & pasting & reusing software from all over the web. When you're working professionally you might be confronted with the fact that not all open-source libraries are as free, as in libre (or as in anarchy) as you would like them to be.

(btw I'm not a lawyer, but I do understand language and code)

The famous GPL requires you to license your derivative works with the same GPL as well. Not only your modifications to the library you are using need to be open-sourced, but also the code that you wrote using that other project as a library without which your project wouldn't work. …