Open source code is software where the underlying instructions are published openly so anyone can read, modify, and share them

When software is open source, the original code — the human-readable instructions that tell a program what to do — is made available to the public. You can download it, study it, change it, and redistribute your changes. This is different from closed source or proprietary software, where the code stays locked away and only the company that built it can modify it.

Open source doesn't mean the software is free to use, though many open source programs are. It means the source code itself is transparent. Anyone with programming knowledge can see exactly how the software works, spot security problems, or adapt it for their own needs. The person or organization that created it typically licenses it under specific terms that explain what you can and cannot do with it.

You've probably used open source software without realizing it. Linux (an operating system), Firefox (a web browser), and WordPress (website software) are all open source. So are many tools that power websites and apps behind the scenes.

Key Takeaways

  • Open source means the code is publicly available for anyone to read and modify, not that it costs nothing or has no restrictions.
  • Each open source project has a license that sets the rules — some require you to share your changes, others don't.
  • Open source software is often more secure because many people can review the code and report problems.
  • Closed source software keeps its code private, so only the company can fix bugs or add features.
  • You can use open source code in your own projects, but the license determines what you must do in return.

How open source licenses work

Open source code comes with a license — a legal document that explains what you're allowed to do with it. The license is not optional. It's the agreement between you and whoever created the code. Different licenses have different rules, and choosing the wrong one for your project can create legal problems.

Some licenses, like the MIT License and Apache License 2.0, are permissive. They let you use the code almost however you want — including in commercial software or closed source projects — as long as you include a copy of the license and give credit to the original author. You don't have to share your changes.

Other licenses, like the GNU General Public License (GPL), are copyleft. They require that if you modify the code or include it in your own software, you must release your version under the same license. This means your changes must also be open source. Copyleft licenses are stricter but ensure the code stays open for everyone.

Before you use any open source code, read its license. Most projects display it in a file called LICENSE in the main folder. If you're using open source in a business or commercial project, understanding the license is especially important.

Why companies and developers use open source

Open source saves time and money. Instead of writing everything from scratch, developers can use existing code that's already tested and maintained by a community. A company building a website might use open source libraries for common tasks like processing payments or handling user logins, rather than building those features themselves.

Open source also builds trust. Because the code is visible, security researchers and other developers can review it and report problems. A closed source program might have a hidden security flaw that nobody catches for years. An open source project with active users and reviewers is more likely to have bugs found and fixed quickly.

Many open source projects are maintained by volunteers or small teams who believe in sharing their work. Others are backed by large companies like Google, Microsoft, or Meta, which use open source to build community around their tools and reduce development costs. Some companies release open source versions of their software to attract users, then sell support or premium features on top.

The difference between open source and free software

Open source and free software are related but not the same thing. Free software emphasizes freedom — the right to run, study, modify, and share the code. Open source emphasizes the practical benefits of having code available: better security, faster development, and community collaboration. The distinction matters mostly to developers and organizations with strong philosophies about software freedom.

In practice, most projects that are open source are also free to use. But you can have open source software that costs money (you pay for support or hosting), and you can have free software that's closed source (the company gives it away but keeps the code private). The license determines what's actually allowed.

Common misconceptions about open source

Open source does not mean unsupported or low-quality. Many open source projects are maintained by professional developers and used by major companies. Linux powers most of the internet's servers. Open source doesn't mean abandoned either — active projects have regular updates, bug fixes, and new features. Inactive projects do exist, so check when the last update was before relying on something.

Open source also doesn't mean you can do whatever you want with the code. The license sets boundaries. You might not be able to use it in a commercial product, or you might have to release your own code if you do. Read the license first.

Finally, open source is not the same as public domain. Public domain code has no copyright and no restrictions. Open source code is copyrighted, but the copyright holder has chosen to share it under specific terms. Those terms still apply.

How to find and use open source code

The largest repository of open source code is GitHub, a website where millions of projects are hosted. You can search by programming language, topic, or project name. Other repositories include GitLab, Bitbucket, and SourceForge. Most projects include a README file that explains what the code does, how to install it, and how to use it.

When you find a project you want to use, download it or copy the code into your own project. If the project is actively maintained, you can also "watch" it to get notifications about updates. Many open source projects accept contributions — if you fix a bug or add a feature, you can submit your changes back to the original project for others to benefit from.

Before using open source code, check three things: the license (to make sure it's compatible with your project), the last update date (to see if it's still maintained), and the documentation (to understand how to use it). If a project hasn't been updated in years and you need support, you might be on your own.

Open source vs. closed source: what you need to know

Closed source software is proprietary — the company owns the code and doesn't share it. You can use the program, but you can't see how it works, modify it, or redistribute it. Microsoft Windows, Adobe Photoshop, and most commercial software are closed source. The company controls all updates and fixes.

Open source gives you transparency and flexibility. Closed source gives you a single point of support — you contact the company if something breaks. Both models have trade-offs. Open source projects can disappear if the maintainers lose interest. Closed source software can become obsolete if the company stops supporting it or goes out of business. Many organizations use a mix of both, choosing open source for some tools and closed source for others based on their needs.

Frequently Asked Questions

Can I use open source code in my commercial product?

It depends on the license. Permissive licenses like MIT and Apache allow commercial use as long as you include the license and credit the original author. Copyleft licenses like GPL require you to release your product under the same license, which means your code must also be open source. Always check the license before using open source code in a business project.

Do I have to pay for open source software?

Most open source software is free to download and use. Some projects ask for donations or offer paid support and hosting. You're not paying for the code itself — you're paying for services like technical support, hosting, or premium features built on top of the open source foundation.

Is open source software less secure than closed source?

Not necessarily. Open source can be more secure because many people review the code and report problems. However, security depends on how actively the project is maintained and how many people are actually reviewing it. A well-maintained open source project is often more secure than closed source software with a small development team.

What happens if an open source project stops being maintained?

If a project becomes inactive, you can still use the code, but you won't get updates or bug fixes. Some projects are "forked" — developers copy the code and start a new project with their own improvements. You can also hire developers to maintain it yourself or switch to an actively maintained alternative.

Can I modify open source code and keep my changes private?

It depends on the license. Permissive licenses allow you to modify the code and keep changes private. Copyleft licenses require you to share your modifications if you distribute the software. If you modify code but only use it internally, most licenses allow you to keep changes private.