(paraphrased from the “What does free mean?” discussion on Debian.org)
Free means “at no cost”. However, it also means at least twenty other things that have nothing to do with cost.
Things like “liberty” and “lack of constraint”. When Debian says “free”, Debian means freedom, not mere price.
You might say it’s like spring water. There’s bottled spring water that you may pay a pretty penny for particularly those with designer labels or near mythic reputations for their medicinal properties. And then there is spring water you discover while out hiking in the pristine wilderness that is so naturally refreshing that it doesn’t seem real. Tasty, delicious spring water from rain and snow filtered naturally through rock that has flowed for some thousands-of-years old source creating nature’s own perfect water. And it’s free!
How about free software that you cannot pass along? Improve? How about a piece of software designed by a company specifically to kill off a smaller rival? What about so-called “free” software that only exists to push another product, or to advertise its more attractive functions and generations for which you will most certainly have to pay, now or at a later date.
What about software that begins free, in the public domain, which is then used for part of non-free software? You can consider any further improvements lost to society. Only copyrighted and licensed software can remain truly free.
A copyright is designed to protect the rights of a work’s creator. Licenses, on the other hand, are designed to allow use of a work in ways (defined within the license terms) that its creator finds to be acceptable.
Software companies tend to restrict use as much as possible. Authors of free software, on the other hand, require that the code not be used in software that will have restricted use down the line (i.e., proprietary software). This must include the source code that composes the software., so that anyone may fix bugs or make any other sort of improvement.
Writing one’s own license has certain drawbacks, not least of which is an ignorance of the way in which the terms can be interpreted in a legal sense. The opposite drawback is equally dangerous — i.e., knowing exactly how the terms can be interpreted and writing them in such a way that will benefit yourself.
Licenses have been developed (with foresight as well as trial by fire), including the GNU GPL, which is the most commonly-used free license. Perl has also developed the “Artistic License” (which may or may not be a play on words). Other licenses exist, the best of which share certain elements in common.
The Debian Free Software Guidelines (DFSG) were developed for just that purpose, and can be viewed on the “Debian Social Contract” page of this site or the original Debian site.