This article firstly talks about GNU project. Secondly, it explains the four freedoms of free software, and then introduces the concept and characteristics of copyleft. In addition, it analyzes essential contents of GPL (GNU General Public License) v2, such as action conditions for copying, distributing, and modifying a program, and the infection of GPL to modified program, etc. Finally, it introduces two famous cases related to GPL: SCO vs. IBM lawsuit, and the case about Harald Welte's preliminary injection request against Sitecom's German subsidiary. It briefly talks about the upgrade to GPLv.3 as well.
p.s. GPL v.3 had not yet been released while this articles finished. Thus, this article talks less about GPL v.3