tl;dr: Discourse software is released under the terms of the GNU General Public License (GPL-2.0-or-later). This documentation informs about the reasons and implications of this choice. Remember: “There is only one version of Discourse – the awesome open source version.”
In February 2013, @diego was "[c]urious about “why GNU [license] and not something else, such as BSD?”.
Why Pick the GNU General Public License?
Implications of the GPL
This section provides an overview of common questions we’ve had in the past regarding the GPL. For more information, please refer to the more exhaustive Copyleft and the GNU General Public License: A Comprehensive Tutorial and Guide.
Can I embed Discourse in my website?
Can I make modifications to Discourse without contributing them back?
Yes you can, for your own usage. But if you distribute modified sources or binaries you must do so under the terms of GPL-2 §2.
Read on to the next two questions.
Can I include features for my own website without distributing the code?
Should I release my Discourse plugins under the terms of the GPL?
Will Discourse remain GPL forever?
Why not AGPL?
Why choose a restrictive license?
Discourse Trademark, Logos, Artwork
Discourse is a registered trademark of Civilized Discourse Construction Kit.
Discourse logo and “Discourse Forum” ®, Civilized Discourse Construction Kit, Inc.