Here's is Jeremy Blum's articulation of the community standards associated with GNU GPL.

• Open Source License
  -------------------
  * All Code Copyright 2013 Jeremy E. Blum, Blum Idea Labs, LLC.
  * This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
  * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
  * A copy of the GNU General Public License is included along with this software. It can also be found at (http://www.gnu.org/licenses/).
• Sharing
  -------
  Under the GNU GPL, you are free to do whatever you want with this provided code. However, I'd really appreciate it if you could do the following things when re-using any code that I have released for this book:
  * Provide attribution at the top of your code, like this "This code adapted from code by Jeremy Blum (www.jeremyblum.com), for the "Exploring Arduino" book (www.exploringarduino.com). Its use and modifcation are permitted under the terms of the GNU GPL."
  * If you make something awesome with the help of this book and/or my code examples, please let me know - I'd love to hear about it! You can hit me up on [twitter](http://www.twitter.com/sciguy14) or via [my website](http://www.jeremyblum.com/contact).
  * Share! If you make something awesome, please consider releasing it as an open source project so other people can learn from what you've done.