What Is Free and Open Source Software?

Free and Open Source Software (FOSS) is software released under a license that guarantees four essential freedoms: to run the program for any purpose, to study how it works, to modify it, and to redistribute copies — with or without your changes. When a program meets these conditions, nobody can take it away from you, silently change it, or lock it behind a subscription.

The word free refers to freedom, not price. Plenty of free-of-charge apps exist that are not free software — they show ads, collect your data, or refuse to export your own files. The real difference is control: with FOSS the source code is public, so the program can be inspected, audited, and improved by anyone. Development is usually a collaborative effort of volunteers, companies, and foundations rather than a single vendor.

Licenses make this possible, and they come in two broad families. Copyleft licenses such as the GNU GPL require that derivatives stay free, which keeps the ecosystem open as it grows. Permissive licenses such as MIT, Apache 2.0, and the BSD licenses allow code to be reused almost anywhere, including inside proprietary products. Both approaches produce free software; they simply answer the question "how should the code travel?" differently.

You already rely on FOSS every day, whether you know it or not. Linux runs most of the internet's servers, Android is built on the Linux kernel, and programs like Firefox, VLC, LibreOffice, GIMP, and WordPress are all free software. Entire operating systems — Ubuntu, Fedora, Debian, Linux Mint — can completely replace Windows or macOS.

That is why FOSS matters for ordinary users, not just developers. It means software that respects your privacy, works for you instead of an advertising platform, and cannot disappear because a company lost interest. The rest of this section looks at these ideas in detail.