Open Source Software (OSS) is software released under a specific license that allows the source code to be accessible to the public. The end users are granted the right to use, study, modify and distribute the software and source code.

"'Free software' is a matter of liberty, not price. To understand the concept, you should think of 'free' as in 'free speech,' not as in 'free beer'."
—Richard M. Stallman

The following is the Open source definition which is criteria that open source software must comply with. It was written by the Open Source Initiative and was originally derived from Debian Free Software Guidelines.

1)Free Redistribution

“The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources. The license shall not require a royalty or other fee for such sale.”

Simplified – You can sell or give away the software as a component and doesn’t have to pay royalty or any fees.

2)Source Code

“The program must include source code, and must allow distribution in source code as well as compiled form. Where some form of a product is not distributed with source code, there must be a well-publicized means of obtaining the source code for no more than a reasonable reproduction cost, preferably downloading via the Internet without charge. The source code must be the preferred form in which a programmer would modify the program. Deliberately obfuscated source code is not allowed. Intermediate forms such as the output of a preprocessor or translator are not allowed.”

Simplified – The methods to access the program’s source code must be easy.

3)Derived Works

“The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.”

Simplified – Changes and derivative of the software must be allowed.

4)Integrity of The Author's Source Code

“The license may restrict source-code from being distributed in modified form only if the license allows the distribution of "patch files" with the source code for the purpose of modifying the program at build time. The license must explicitly permit distribution of software built from modified source code. The license may require derived works to carry a different name or version number from the original software.”

Simplified – Derivatives must be clearly and officially allowed to be distributed and may be required to be recognized separately from its base source.

5)No Discrimination Against Persons or Groups

“The license must not discriminate against any person or group of persons.”

Simplified – Anyone can use the software.

6)No Discrimination Against Fields of Endeavor

“The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.”

Simplified – Any party is allowed to use the software, no matter the endeavors of the said party.

7)Distribution of License

“The rights attached to the program must apply to all to whom the program is redistributed without the need for execution of an additional license by those parties.”

8)License Must Not Be Specific to a Product

“The rights attached to the program must not depend on the program's being part of a particular software distribution. If the program is extracted from that distribution and used or distributed within the terms of the program's license, all parties to whom the program is redistributed should have the same rights as those that are granted in conjunction with the original software distribution.”

Simplified - The rights of the original distribution should be the same rights granted to redistributed programs extracted from a software distribution.

9)License Must Not Restrict Other Software

“The license must not place restrictions on other software that is distributed along with the licensed software. For example, the license must not insist that all other programs distributed on the same medium must be open-source software.”

Simplified – An open source software and a commercial software can be distributed alongside each other without restrictions.

10)License Must Be Technology-Neutral

“No provision of the license may be predicated on any individual technology or style of interface.”

Simplified – The license should not be based on any single technology.

The source code for OSS is generally stored in a public repository in technologies such as GitHub and GitLab. These repositories are accessible to anyone but are usually used by developers and likeminded programmers. These individuals may use their skills to modify the source code for their personal projects or to contribute improvements to the current project. Open source software are maintained and improved by a community with members all around the world. These members may vary from students to multinational corporations whom all benefits from the innovation and sharing of knowledge to better improve the project.

OSS comes with a license with terms on how developers/users can use, modify and distribute the software.

The most used licenses are:

MIT License

GNU General Public License (GPL) 2.0

Apache License 2.0

GNU General Public License (GPL) 3.0

BSD License 2.0 (3-clause, New or Revised)

Some popular open source software projects include:

Linux operating system

Wikipedia

Firefox

Wordpress

Moodle

VLC media player

The apache software

GIMP

Blender

Free Software Foundation

Software Freedom Conservancy

Linux Foundation

Apache Software Foundation

Eclipse Foundation

Mozilla

The Cathedral & The Bazaar - (1999) Eric S. Raymond

Opensource.org