Free software

Free Software runs the world
Of the world's five hundred fastest supercomputers, 494 (98.8%) use the Linux kernel. The world's second fastest computer is the Oak Ridge National Laboratory's Titan supercomputer (illustrated), which uses the Cray Linux Environment.

Free software, freedom-respecting software, or software libre is computer software distributed under terms that allow the software users to run the software for any purpose as well as to study, change, and distribute the software and any adapted versions. Free software is a matter of liberty, not price: users, individually or collectively, are free to do what they want with it, including the freedom to redistribute the software free of charge, or to sell it, or charge for related services such as support or warranty for profit.

The right to study and modify software entails availability of the software source code to its users. While this right is often called 'access to source code', but the Free Software Foundation recommends to avoid using the word 'access' in this context because it is misleading and may make people believe that they may have a copy of the source code unconditionally. This right is only conditional on the person actually having a copy of the software, i.e. being a software user.

Richard Stallman used the already existing term free software when he launched the GNU Project—a collaborative effort to create a freedom-respecting operating system—and the Free Software Foundation (FSF). The FSF's Free Software Definition states that users of free software are free because they do not need to ask for permission to use the software.

Free software thus differs from

For computer programs that are covered by copyright law, software freedom is achieved with a software license by which the author grants users the aforementioned freedom. Software that is not covered by copyright law, such as software in the public domain, is free if the source code is in the public domain, or otherwise available without restrictions.

Proprietary software, including freeware, use restrictive software licences or EULAs and usually do not provide access to the source code. Users are thus prevented from changing the software, and this results in the user relying on the publisher to provide updates, help, and support. This situation is called vendor lock-in. Users often may not reverse engineer, modify, or redistribute proprietary software. Other legal and technical aspects, such as software patents and digital rights management may restrict users in exercising their rights, and thus prevent software from being free. Free software may be developed collaboratively by volunteer computer programmers or by corporations; as part of a commercial, for-profit activity or not.