Open-source software is software whose source code anyone may read, change and share, because its licence gives everyone those rights. You can usually download and use it without paying for a licence, and you are free to see exactly how it works.
You probably already use some. The Firefox browser, the VLC media player and the WordPress website builder are all open source. So are many of the tools in our directory, such as Umami, which shows how people use a website.
In this guide we look at what open source means, one step at a time. You will see what source code is, why the licence is what makes software open source, how open source differs from closed-source and “free” software, and some well-known examples. We finish with the honest trade-offs and a simple way to check whether a project is really open source.
Summary
- Open source is about rights, not just visible code. The licence must let anyone use, study, change and share the software, for any purpose.
- Source code is the human-readable version of a program. Having it lets people check how the software works, fix it and improve it.
- The Open Source Initiative keeps the official definition. It also lists the licences that meet it, such as MIT, Apache 2.0 and the GPL.
- Closed-source (proprietary) software keeps its code private. Only its owner can legally change it, and you use it on the owner’s terms.
- “Free software” means freedom, not price. It covers almost the same programs as open source, with a stronger focus on users’ freedom.
- Free to use is not always free to run. The licence costs nothing, but hosting, paid services and your time may not.
- Check the licence to be sure. Look for a LICENSE file and compare it with the Open Source Initiative’s list.
What is source code?
Source code is the set of written instructions that a program is built from. Programmers write it in a programming language, which people can read. A computer then turns it into a version it can run, which people cannot easily read.
Think of source code as a recipe and the program as the finished dish. With only the dish, you can eat it but you cannot easily tell how it was made. With the recipe, you can see every ingredient, cook it yourself, adjust it to your taste and share your version with friends.
That is why access to source code matters. It lets anyone check what a program really does, fix problems without waiting for the original maker, and build improvements on top of it.
What makes software open source? The licence
Being able to see the code is not enough on its own. As the Open Source Initiative (OSI), the organisation that maintains the official definition, puts it: “Open source doesn’t just mean access to the source code.”
What makes software open source is its licence: the legal permission that says what you may do with the software. The OSI’s Open Source Definition sets out ten conditions a licence must meet. In plain English, the most important ones are:
- Anyone may share it. The licence must not stop anyone from selling or giving away the software.
- The source code must be available. The program must include its source code, or say where to get it.
- Changes are allowed. You must be allowed to modify the software and share your changed version.
- No person or group may be excluded. The licence cannot single out anyone.
- No use may be excluded. The OSI’s own example: a licence “may not restrict the program from being used in a business, or from being used for genetic research.”
The OSI reviews licences against this definition and publishes the ones that pass in its list of OSI-approved licences. Three of the best known are the MIT License, the Apache License 2.0 and the GNU General Public License (GPL). They all meet the definition, but their conditions differ, for example in what you must do when you share a changed version, so it is worth reading the licence of any project you plan to change and redistribute.
Open source vs closed source
Software that is not open source is usually called closed source or proprietary: its owner keeps the source code private and decides what you may do with the program.
| Open source | Closed source (proprietary) | |
|---|---|---|
| Can you see the source code? | Yes | No, only the owner can |
| Can you change it? | Yes, the licence allows it | No |
| Can you share copies? | Yes | Only if the owner allows it |
| What does the licence cost? | Usually nothing | Often a purchase or subscription |
| Who provides support? | The community, and sometimes a company that sells support | The company that owns it |
| Who decides its future? | The maintainers, and anyone can continue it from the code | The owner alone |
Neither model is automatically better. Many people use both every day: a closed-source operating system with an open-source browser, for example. The difference that matters most is control. With open source, you and everyone else have the right to keep using, fixing and sharing the software, even if the original maker stops.
Free software vs open source
You will often see the term free software, which comes from the Free Software Foundation. Here, “free” is about freedom, not price. In the foundation’s words, think of “free” as in “free speech,” not as in “free beer.”
The foundation defines free software by four freedoms:
- Freedom 0: to run the program as you wish, for any purpose.
- Freedom 1: to study how the program works and change it. Access to the source code is needed for this.
- Freedom 2: to share copies so you can help others.
- Freedom 3: to share your changed versions with others.
So what is the difference from open source? In practice, the two terms cover almost the same programs. The Free Software Foundation itself says open source means “more or less the same category as free software”, but not exactly: each side has accepted a few licences the other has not. The bigger difference is the message. “Free software” stresses users’ freedom as a principle; “open source” stresses the practical benefits of sharing code. You will also see the combined term FOSS (free and open-source software).
Two things are easy to mix up:
- Free software can be commercial. The Free Software Foundation says “Free software” does not mean “noncommercial”: a free program must be available for commercial use.
- “Freeware” is not free software. The foundation describes freeware as programs you may share but not change, usually without their source code.
Is open-source software free to use?
Usually the licence costs nothing: you can download and use open-source software without paying the people who wrote it. But “free to download” is not always “free to run”.
- Some software needs somewhere to run. Website tools like Umami run on a server, which you rent or provide yourself. Our guide What Is Self-Hosting? explains those costs.
- Some software depends on paid services. OpenSEO, an open-source search-research tool, is free to install, but the search data it uses comes from a paid supplier.
- Some projects sell a hosted version or support. Umami offers an online version with a free plan, and OpenSEO an online version with a subscription, alongside the free software you can install yourself.
Programs you install on your own computer are often genuinely free to use. PhotoCraft, an open-source image editor, runs on your Mac, Windows or Linux computer with nothing to pay.
Examples of open-source software you may already use
Each of these describes itself as open source on its official site:
- Firefox, the web browser. Mozilla says “All Mozilla software is open source and free software,” released under its Mozilla Public License.
- VLC, the media player. VideoLAN calls it “a free and open source cross-platform multimedia player and framework.”
- LibreOffice, the office suite for documents, spreadsheets and presentations. It describes itself as “free and open source software”, with its code under the Mozilla Public License 2.0.
- WordPress, the website and blog builder that WordPress.org calls the platform of choice for millions of sites. It states that “WordPress is open source software,” licensed under the GPL version 2 or later.
- Linux, the kernel (the core part) of the Linux family of operating systems. Its documentation states that the Linux kernel is provided under the GNU General Public License version 2.
- Blender, the 3D modelling and animation suite. It calls itself “The Free and Open Source 3D Creation Suite”, licensed under the GPL.
Our directory adds more, with what each one costs to run and how to install it. The projects in this guide are at the end of the page, and you can browse all projects.
Why people choose open source, and the trade-offs
Reasons people choose it:
- You can see how it works. Anyone can inspect the code, so it is harder to hide what a program does.
- You are not tied to one company. If the original maker changes direction, raises prices or stops, the code is still there for others to continue.
- There is often no licence fee. That can make a real difference for individuals, charities and small businesses.
- You can adapt it. If you have the skills, or pay someone who does, you can change it to fit your needs.
The trade-offs to weigh honestly:
- Support varies. Some projects have large communities or companies offering paid support; others are run by one or two volunteers.
- Polish varies too. Some open-source apps are as easy to use as any commercial product; others assume more technical knowledge.
- Projects can slow down or stop. Before you rely on one, look at how recently it was updated. Every project page in our directory shows its latest release and recent activity.
- Open is not automatically secure, and closed is not either. Open code can be checked by anyone, but only if people actually check it. Look at whether the project is maintained and fixes problems promptly.
How to check whether a project is really open source
Many apps call themselves “free” or “open”, so it is worth checking. It takes a few minutes:
- Find the licence. On the project’s website or code page (often on GitHub), look for a file called LICENSE or COPYING, or a “License” line. If you cannot find a licence, do not assume you may reuse the code: ask the project first.
- Compare it with the OSI’s list. If the licence appears in the OSI-approved licences, the software is open source by the official definition.
- Watch for limits. Some projects publish their code but with a licence that forbids certain uses, for example business use. They may be useful, but by the Open Source Definition they are not open source, because an open-source licence may not exclude any field of use.
- Do not rely on “free download”. Software that is free to download may still be closed source. The Free Software Foundation notes that “freeware” usually comes without its source code.
Our project pages show each project’s licence as GitHub reports it, so you can see it at a glance before you dig deeper.
Frequently asked questions
Is open-source software safe?
It can be, but being open source does not make software safe on its own. Anyone can inspect open code, which helps problems get found. What matters most is whether the project is actively maintained and fixes problems quickly, so check its recent activity before you rely on it.
Can I use open-source software for my business?
Yes. The Open Source Definition says a licence may not restrict the program from being used in a business, and the Free Software Foundation says free software must be available for commercial use. Licences do differ in what they ask when you share a changed version, so read the licence if you plan to modify and redistribute the software.
Who pays for open-source software?
Many people and organisations, in different ways. Some projects are built by volunteers, others by companies or foundations. Some earn money by selling a hosted version or support, as Umami and OpenSEO do with their online versions.
Is open source the same as free?
Not exactly. Open-source software usually has no licence fee, but running it can still cost money, for example for a server or a paid data service. And “free software” in the Free Software Foundation’s sense means freedom, not price.
What is an example of open-source software?
Well-known examples include the Firefox browser, the VLC media player, the LibreOffice office suite, WordPress, Linux and Blender. Our directory lists more, including Umami, OpenSEO, PhotoCraft and VectorCraft.
