Richard Best has a terrific guide to GPL for WordPress developers along the lines of the famously simple “human readable” Creative Commons license explanations on his site, WP and Legal Stuff, in his post titled “A human readable summary of the GPL“. He actually has two versions, both of which are worth taking a look at. I like his version modelled on the CC license explanation format:
Best has also published “A Practical Guide to WordPress and GPL” and it is available in three packages. The top package is the “business package” which includes –
The standalone ebook option is a little pricey at $25 for the PDF but if you consider the cost of legal advice on the topic, it is probably worth it.