Free software is open source by definition. Open source software is not necessarily free. That sounds like a semantic distinction, and it stops being one the moment you build a product on somebody else’s component and read what its licence obliges you to publish.
Why copyright makes this possible
Copyright is unusual among IP rights: it vests on creation, with no registration required for protection. That immediacy is what makes copyleft and permissive licensing work — the creator holds rights the moment the work exists, and can therefore set the terms on which others may use it.
What free software means
“Free” refers to freedom, not price — which is why it is often called libre software.
For software to qualify as free, it must grant users four freedoms:
- the freedom to run the program, for any purpose;
- the freedom to study how it works and change it to suit your requirements;
- the freedom to redistribute copies; and
- the freedom to distribute copies of your modified versions to others.
These were conceptualised by Richard Stallman of the Free Software Foundation, which created and maintains the GNU General Public License. A program must grant all four to be free software.
Because “free” means freedom, rights holders are not barred from charging for free software. The FSF actively encourages charging what you think appropriate.
Free versus open source
The terms are used interchangeably, and FOSS has gained currency — but there is a real distinction.
Open source means the source code is open to view and modify — satisfying the second freedom. Free software is open source by definition. But open source software need not be free: being able to view source code does not automatically confer the freedom to modify, distribute or incorporate it into another product. Those rights depend entirely on the licence terms.
The software community tends to frame the difference as code quality (the open source movement’s emphasis) versus user rights (the free software movement’s). Legally, the distinction matters most when licensing your own product or building a product using an open source component.
The open source movement gained traction commercially by emphasising security, transparency and cost reduction. Modern computing runs on it — the Linux code behind the great majority of the world’s servers, the Android operating system on most of the world’s smartphones. But visible source code alone does not make something free software; that requires the freedom to modify and distribute derivatives, even for a fee.
The licences
Software that is open source or free, rather than proprietary, is licensed either through copyleft or permissive licences.
A permissive licence guarantees the work is free in the sense above, with minimal restrictions — often called anything goes licences, frequently requiring only that a copy of the licence accompany any distribution.
Copyleft licences:
- GNU General Public License (GPL) — the most popular open source licence and a strong copyleft licence, with an Affero variant for SaaS.
- Mozilla Public License (MPL) — a weak copyleft licence bridging strong copyleft and permissive terms.
- Common Development and Distribution License (CDDL) — weak copyleft, requiring disclosure of source only for the components licensed under it.
- Eclipse Public License — weak copyleft, balancing commercial interests against user rights.
Permissive licences:
- BSD — extremely permissive, among the earliest open source licences.
- MIT — extremely permissive, requiring only that the copyright notice and licence be included in distributions.
- Apache — modern, widely adopted for its permissiveness and corporate-friendly approach, including an express patent grant.
Why the choice matters
The right licence depends on your target audience, the intended purpose of the software, and — critically — whether you have used somebody else’s free code in your product.
That last point is where most commercial problems originate. Strong copyleft propagates. Incorporating GPL-licensed code into a distributed product can oblige you to release your own source under the same terms. Weak copyleft licences confine that obligation to the licensed components themselves. Permissive licences generally impose no such requirement at all.
For a business intending to keep its source proprietary, the difference between incorporating an MIT-licensed library and a GPL-licensed one is not a matter of preference — it determines whether the product can ship on the terms you planned.
The takeaways
- Four freedoms define free software — run, study and modify, redistribute, distribute modifications.
- Open source does not imply free — visible code is not permission to reuse.
- Strong copyleft propagates to products that incorporate the code.
- Audit components before shipping — the licence, not the label, governs.
Frequently asked questions
What is the difference between free and open source software? Free software grants four specific freedoms including modification and redistribution. Open source means the code is visible, which does not by itself confer those rights.
Can free software be sold? Yes — “free” refers to liberty, not price, and the Free Software Foundation encourages charging what the author considers appropriate.
What does strong copyleft mean for my product? Incorporating strongly copyleft code, such as GPL-licensed code, into a distributed product can require you to release your own source on the same terms.
Which licences are most permissive? BSD, MIT and Apache — typically requiring little more than retention of the copyright notice and licence text.