The Rise of Open Source

The basic technique of open source is to assert copyright, and grant a public license that permits using, copying, modifying, and redistributing software source code and all derivatives (compiled programs, libraries, packages, improved versions) of the source code to anyone. There are many variants, but in general these licenses are lumped together as "permissive licenses".

The free software movement is associated with a variant public license, which is an open source license, but in addition to the permissions granted also requires that redistribution be under the same license terms (including any modifications made by the distributor!) This reciprocal requirement is often called "copyleft".
- Because it must be the same license, there is a "network externality" that different copyleft licenses cannot be imposed at the same time (they make the software unredistributable, or even unusable). The most popular copyleft license by far is the GNU General Public License.
The idea of "open source" has now been extended to
- open hardware (e.g., the "Maker" movement and 3D printers)
- open content (e.g., like Wikipedia)
- open data (Singapore air quality sensor network
- open government documents (FOIA in many jurisdictions)