Free Software vs Open Source vs Copyleft: What the Difference Means
“Free software”, “open source” and “copyleft” are often used as if they mean the same thing. They overlap, but they are different ideas, and the difference decides what you may legally do with a program: use it at work, change it, sell it, or build it into your own product. This article explains where the movement came from, what each term really means, how the main licence families differ, and what to check before you use or publish code.
Where it started
In the 1960s and 1970s, software was often shared freely among researchers, usually with source code included. By the early 1980s, most commercial software was distributed only as binaries under restrictive licences. In 1983, Richard Stallman announced the GNU Project to build a complete Unix-compatible operating system made entirely of software users are free to run, study, change and share. In 1985 he founded the Free Software Foundation (FSF). GNU’s tools were later combined with the Linux kernel (started by Linus Torvalds in 1991) to form the systems we call Linux distributions.
What “free software” means
“Free” here means freedom, not price (“free as in speech, not as in beer”). The FSF defines free software as respecting four essential freedoms:
- Freedom 0: run the program for any purpose.
- Freedom 1: study how it works and change it (which requires access to the source code).
- Freedom 2: redistribute copies.
- Freedom 3: distribute copies of your modified versions.
Free software may be sold; the freedoms are about what users can do with the program. (An older version of this article said free software has “no limitations”. That is not accurate: free-software licences do have conditions, and copyleft licences in particular have strong ones.)
What “open source” means
The term open source was proposed by Christine Peterson in February 1998, and people including Eric Raymond and Bruce Perens then founded the Open Source Initiative (OSI) to promote it as a more business-friendly label. The OSI maintains the Open Source Definition and approves licences that meet it. Its criteria include free redistribution, source code availability, permission to make derived works and no discrimination against people or fields of use. In practice, almost all free software is open source and vice versa, but the two communities emphasise different things: free software stresses user freedom as an ethical goal, while open source stresses the practical advantages of open development.
Copyleft: using copyright to keep software free
Copyleft is a licensing technique. The author keeps the copyright but grants everyone the freedoms above on one condition: if you distribute the software or a modified version, you must give recipients the same freedoms, which includes offering the source code under the same licence. That prevents someone from taking a free program, improving it and releasing it as closed, proprietary software.
The best-known copyleft licence is the GNU General Public License (GPL), used by the Linux kernel (version 2) and many GNU tools. Its obligations apply when you distribute: running a modified GPL program privately does not require you to publish anything.
The main licence families
| Type | Examples | What you must do | Can it be used in proprietary products? |
|---|---|---|---|
| Permissive | MIT, BSD, Apache 2.0 | Keep the copyright and licence notice (Apache 2.0 adds notice and patent terms) | Yes |
| Weak copyleft | LGPL, MPL 2.0 | Share changes to the licensed files or library itself | Yes, when it is linked or used as required by the licence |
| Strong copyleft | GPL v2, GPL v3 | Distribute the source of the combined work under the GPL | If you distribute a combined program, the whole must be under the GPL. Private use and independent, separate programs shipped alongside are different cases. |
| Network copyleft | AGPL v3 | As GPL, and also offer source to users who use it over a network | If you modify an AGPL program, you must offer its source to the users who interact with it over the network. Commercial hosting is allowed, and separate programs are not automatically covered. |
This is a simplified overview, not legal advice. Licence compatibility and what counts as a “combined work” can be complicated. If you are building a product or company, ask a lawyer.
Why it matters in practice
- For users and admins: free and open-source software can be inspected, patched and self-hosted. That is why much of the internet’s infrastructure (Linux, Apache, nginx, PostgreSQL, MySQL, Kubernetes) is open source. This site itself runs on WordPress, which is GPL-licensed.
- For developers: choosing a licence decides how others can reuse your work. Permissive licences maximise adoption; copyleft keeps improvements shared.
- For companies: keep an inventory of open-source components and their licences (a software bill of materials). Being non-compliant can force you to release source code or stop distributing.
Open source does not automatically mean secure, either. Many eyes can review code, but only if someone actually does, which is why supply-chain problems still occur in popular projects. Keep your dependencies updated and check where they come from.
Common questions
Is open-source software always free of charge?
Often it is, but not necessarily: companies sell support, hosting and packaged versions of open-source software, and the licences allow selling copies.
Is Linux free software or open source?
Both. The Linux kernel is licensed under GPL version 2, and most distributions include a mix of GPL and permissively licensed software. The history of Linux shows how the kernel and GNU tools came together.
Can I use GPL code in my company’s internal tools?
Yes. The GPL’s requirements apply when you distribute the software outside the organisation, not when you use it internally.
Keep reading
- History of Linux: From a 1991 Hobby to Running the Cloud — How the Linux story began.
- Linux Distributions Explained: How to Choose the Right Distro — Choosing a distribution.
- 12 Examples of Linux in Everyday Life (You Use It Daily) — Open source in devices you already use.
