Free Software vs Open Source: What's the Difference?
They build the same software and argue about why. The real difference between free software and open source is a difference of motive: freedom as a right, or openness as a better method.
People use "free software" and "open source" as if they were the same thing. Most of the time the code is identical. The difference is not technical at all. It is a difference of why, and that why has shaped four decades of how software gets built.
Free software: freedom as a right
The older idea is free software, and the key word is freedom, not price. As the slogan goes, "free as in freedom, not free as in beer." It comes from Richard Stallman and the Free Software Foundation, and its argument is moral: that users deserve four freedoms, to run a program, to study it, to change it, and to share it. Anything less, in this view, is an injustice done to the user. You can read the founding statement in our exhibit on the GNU Manifesto.
Free software enforces those freedoms with a clever license trick called copyleft, embodied in the GPL: you may use and modify the code, but if you distribute it, you must pass the same freedoms on. The freedom is made viral, so it cannot be quietly taken away.
Open source: openness as a method
In 1998, after Netscape released its browser's source code, a group of people wanted to sell this way of working to businesses, and decided the word "free" was the problem. It sounded like zero-cost, or worse, like anti-commercial activism. So they coined open source: same practice, different pitch. The argument shifted from ethics to engineering, captured in Eric Raymond's essay The Cathedral and the Bazaar. Open development, it claimed, simply produces better, more reliable software, because "given enough eyeballs, all bugs are shallow."
Open source is comfortable with permissive licenses (like MIT or BSD) that let companies take the code, modify it, and never give anything back. To a free-software purist that is a betrayal; to an open-source pragmatist it is the point, because it gets the code adopted everywhere.
Why both, and which won
Here is the irony: they mostly build the same things. Linux, the project that proved the model, is free software under the GPL and the poster child of open source at once. The two camps cooperate constantly and disagree constantly.
If you measure by adoption, open source "won": the world's infrastructure runs on it. If you measure by principle, free software keeps the conscience, the reminder that this was never only about better code, but about who controls the machine. The honest summary: same software, two souls. When someone insists on one term over the other, they are usually telling you which of those souls they care about.