unfortunately, the last poster's link probably leads you down a rabbit hole into miles of confusing twisty little passages (all alike?).
I think I can shed some light on this based on my own experience.
back in the late 80's, borland's Turbo C had a requirement for ALL programs built with their compiler that you distributed: the program HAD to have a copyright statement. Their requirements were simple: an 'informal' copyright statement that looked kinda like this:
Copyright (c) 2014 by Joe Citizen, all rights reserved
the 'all rights reserved' clause is important in some countries.
following this, you could do what you want with it, INCLUDING lumping in "you may do what you want with this" including public domain.
In short, you declare it is YOURS, and you declare what can be done with it on YOUR terms.
As far as I know there has been no challenge to this kind of informal unregistered copyright. DEFENDING IT is, of course, another story. If you're really concerned, contact a copyright/trademark/patent attorney.
but I should at least provide a link to some reasonable resources: http://www.uspto.gov/faq/trademarks.jsp http://copyright.gov/help/faq/index.html
So I hope I have answered this question sufficiently for ANYONE interested in copyrights with respect to their own works.
There are also some resources over at the Free Software Foundation with respect to software licensing, open source, GPL, etc.. They have their own 'copyleft' definition that's similar to copyright but specifically enforces open source-ness. So more research if you want to. This is 'captain obvious' to most of us but worth saying anyway in case someone's new to the intarweb or something.