Four Freedoms (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 or libre software is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versions. Free software is a matter of liberty, not price: users —individually or in cooperation with computer programmers— are free to do what they want with their copies of a free software (including profiting from them) regardless of how much is paid to obtain the program. Computer programs are deemed free insofar as they give users (not just the developer) ultimate control over the first, thereby allowing them to control what their computers are programmed to do.

The right to study and modify a computer program entails that source code —the preferred format for making changes— be made available to users of that program. While this is often called 'access to source code' or 'public availability', the Free Software Foundation recommends against thinking in those terms, because it might give the impression that users have an obligation (as opposed to a right) to give non-users a copy of the program.

Although the term free software had been used loosely in the past,Richard Stallman is credited with tying it to the sense under discussion and starting the Free Software movement in 1983, when he launched the GNU Project: a collaborative effort to create a freedom-respecting operating system, and revive the spirit of cooperation once prevalent among hackers during the early days of computing.

Free software thus differs from

For software under the purview of copyright to be free, it must carry a software license whereby the author grants users the aforementioned rights. 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 too, or otherwise available without restrictions.

Proprietary software uses restrictive software licences or EULAs and usually does 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. Users often may not reverse engineer, modify, or redistribute proprietary software. Additional legal and technical aspects, such as software patents and digital rights management may restrict users in exercising their rights, and thus prevent a piece of software from becoming free.