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The GPL requires that (1) you must ship source or make it available; (2) anyone who receives the program from you can redistribute it further under the GPL; (3) any modified versions that you distribute of a GPL'd program must also be GPL'd. The idea is to make sure that not only is the original GPL'd program free, but everyone can get the source so that they can modify and improve it, and the improved versions are also free. Although the Linux kernel is GPL'd BSD 4.4-lite kernel is not, all the complete programming environments that I know of based on these kernels, including the commercial BSDI system, depend heavily on GPL'd software such as the GNU C compiler. So if you buy a BSDI system, its kernel is proprietary but you're allowed to freely redistribute the C compiler that came with it. For more info, see the GPL itself (distributed with Linux and with all GNU programs such as GNU Emacs), or the article "What is Copyleft?" in the Free Software Foundation newsletter "GNU's Bulletin", or write fsf@gnu.ai.mit.edu. The list has been very active lately--I hope we can keep further traffic on this subject to a minimum. Thanks