Copyright
You are free to assign any copyright terms to your code you wish, within the law, of course. Most authors of Emacs Lisp packages choose to make their software "free" (in availability, not necessarily price) by assigning to it the terms of the GNU General Public License, a special kind of copyright invented by the Free Software Foundation. Software covered by the GPL is assured of remaining freely available, which isn't the case when, say, you release your software into the public domain. (In that case, someone can legally copy your software, make a change to it, call it their own, sell the binaries, and refuse to continue distributing the source code.)
If you wish to place your software under the GPL (a process humorously referred to as "copylefting" your software), you need to include the terms of the GPL either in your source files, or in a separate file (usually COPYING) that is referenced in the copyright notice of each source file (as in the example at the beginning of this appendix). You can see the GPL from within Emacs by typing M-x describe-copying RET.
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