Some of the modules are incomplete. None have documentation, other than what you'll find in the "Description" section of the Update Center wizard and the New Project wizard. However, note that several of them are the final product of some of the tutorials listed on the tutorials page (so you could go there for details). Don't expect to extrapolate best practices from these modules either. A best practice document for NetBeans module development needs to be created at some stage. These samples are useful in showing how to do a lot of pretty practical things with the NetBeans APIs. Again, they're not complete and in some cases you're going to find there are problems with them. That's why these samples are in no way official. The official NetBeans module development samples are the FeedReader and the Paint Application. The NetBeans modules provided here are all just samples resulting from various aspects of the NetBeans APIs that I've personally been working with.
And with that lengthy disclaimer... I wish you a lot of fun with these samples and hope that you'll be able to use them to develop your own modules and rich-client applications!