Source: http://www.gnu.org/events/rms-nyu-2001-transcript.txt
I found this to be a nice analogy for explaining the concept:
<quote>
We are in the free software movement, and I'm going to speak about
what the free software movement is about, what it means, what we have
done, and, because this is partly sponsored by a school of business,
I'll say some things more than I usually do about how free software
relates to business, and some other areas of social life.
Now, some of you may not ever write computer programs, but perhaps you
cook. And if you cook, unless you're really great, you probably use
recipes. And, if you use recipes, you've probably had the experience
of getting a copy of a recipe from a friend who's sharing it. And
you've probably also had the experience -- unless you're a total
neophyte -- of changing a recipe. You know, it says certain things,
but you don't have to do exactly that. You can leave out some
ingredients. Add some mushrooms, 'cause you like mushrooms. Put in
less salt because your doctor said you should cut down on salt --
whatever. You can even make bigger changes according to your skill.
And if you've made changes in a recipe, and you cook it for your
friends, and they like it, one of your friends might say, "Hey, could
I have the recipe?" And then, what do you do? You could write down
your modified version of the recipe and make a copy for your friend.
These are the natural things to do with functionally useful recipes of
any kind.
Now a recipe is a lot like a computer program. A computer program's a
lot like a recipe: a series of steps to be carried out to get some
result that you want. So it's just as natural to do those same things
with computer programs -- hand a copy to your friend. Make changes in
it because the job it was written to do isn't exactly what you want.
It did a great job for somebody else, but your job is a different job.
And after you've changed it, that's likely to be useful for other
people. Maybe they have a job to do that's like the job you do. So
they ask, "Hey, can I have a copy?" Of course, if you're a nice
person, you're going to give a copy. That's the way to be a decent
person.
So imagine what it would be like if recipes were packaged inside black
boxes. You couldn't see what ingredients they're using, let alone
change them, and imagine if you made a copy for a friend, they would
call you a pirate and try to put you in prison for years. That world
would create tremendous outrage from all the people who are used to
sharing recipes. But that is exactly what the world of proprietary
software is like. A world in which common decency towards other
people is prohibited or prevented.
</quote>
