I've still not understood them as well among GPL, BSD or else one. It's quite annoying me. I read a very good article about this. Like the one of "Are you sure you want to use GPL?" which written on Thursday, February 12, 2009. As Armin Ronacher mentioned that:
Recap -- "Before you license your code under an Open Source license: Think about the license! Both types of licenses have their advantages and disadvantages and it would be stupid to use the GPL without thinking just because “everybody does”. Many just do because they haven't read the license either."