Apparently there’s a bit of a row currently going on in the WordPress community, between its founders and the author of a popular WordPress theme, over the terms of the GPLv2 license it’s distributed under. I’ll let the lead WordPress UX designer explain the situation in the simplest terms (my additions in italics):
…the WordPress license states that derivative code (based on WordPress, using WordPress core functions, etc) inherits the GPL license and must retain the user freedoms that the WordPress license guarantees. Chris [author of Thesis, the theme in question] uses WordPress code (in some cases directly copied and pasted from WordPress core), but is not following the rule of the WordPress license, and is instead releasing his Thesis theme under a restrictive license, which takes away the user freedoms that the WordPress license exists to guarantee. Basically, developing on WordPress has one rule by the license agreement: you can take our code for free and build on it, but any work that comes out of that and is publicly distributed must be made available for modification and redistribution just like WordPress itself.
The other thing to understand here is that WordPress is distributed free of charge. Thesis, the theme in question, is sold as a product by its author.
Why is any of this worth mentioning, you ask? Well, many, many organizations use WordPress as an inexpensive content management system to base their websites on. It’s free, it’s flexible, the user base is huge, and there are many ways to extend it beyond a simple blogging platform. This makes it extremely attractive to small web shops and organizations who don’t have tens of thousands of dollars to purchase off-the-shelf solutions or hire developers to write custom software. The argument—and its implications—go beyond a guy selling a theme; it’s a test of the GPLv2 license and open-source software as a whole: Who gets to profit from open-source software and when?