This is a Free Software, Open Source, strong copyleft licence which allows a 12-month grace period between the time you incur the obligation to release the source code of your derived work and the time you actually release the source code.
This graphical explanation is the best introduction to why you should consider using the TGPPL:
http://allmydata.org/~zooko/tgppl.pdf
Read more about this license at http://allmydata.org/source/tahoe/trunk/COPYING.TGPPL.html
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