Open source software is free, in both the political and the economic sense. It can be modified, copied and shared without restrictions. This freedom acts as an open invitation to its users to make improvements.
To maintain this benign state of affairs, open source software is covered by a special legal instrument called the General Public License. Instead of restricting how the software can be used, as a standard software license does, the GPL – often known as a "copyleft" – grants as much freedom as possible. Software released under the GPL (or a similar copyleft licence) can be copied, modified and distributed by anyone, as long as they, too, release it under a copyleft. That restriction is crucial, because it prevents the material from being co-opted into later proprietary products. It also makes open source software different from programs that are merely distributed free of charge. In FSF's words, the GPL "makes it free and guarantees it remains free".
The open source movement has established over the last decade of the twentieth century a new collaborative approach, uniquely adapted to the Internet, to developing high-quality informational products. Initially, its exclusive application was the development of software (GNU/Linux and Apache are among the most prominent projects), but increasingly this collaborative approach was applied to areas beyond the coding of software. One such area is the collaborative gathering and analysis of information, a practice called "Open Source Intelligence".
The most successful open source product is Linux, an operating system created by Finnish student Linus Torvalds in the early 1990s and installed on around 18 million computers worldwide. (Even IBM installs Linux on the computers it sells.) Thousands of volunteers are constantly working on Linux, adding new features and eliminating bugs. Their contributions are reviewed by a panel and the best ones are added to Linux. For programmers, the kudos of a successful contribution is its own reward. The result is a stable, powerful system that adapts rapidly to technological change.
Open source has proved a very successful way of writing software. But it has also come to embody a political stand – one that values freedom of expression, mistrusts corporate power, and is uncomfortable with private ownership of knowledge. It's "a broadly libertarian view of the proper relationship between individuals and institutions. Now some of its supporters are trying out its methods elsewhere. Already there's open source music, open source encyclopaedias, open source law, even open source soft drinks.